Define avifCodecConfigurationBox in internal.h

Move the definition of struct avifCodecConfigurationBox from avif.h to
internal.h. Nothing in avif.h uses this type, so it doesn't need to be
defined in avif.h.
diff --git a/include/avif/avif.h b/include/avif/avif.h
index 5a99884..6974b80 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -660,30 +660,6 @@
 AVIF_API const char * avifCodecName(avifCodecChoice choice, avifCodecFlags requiredFlags);
 AVIF_API avifCodecChoice avifCodecChoiceFromName(const char * name);
 
-typedef struct avifCodecConfigurationBox
-{
-    // [skipped; is constant] unsigned int (1)marker = 1;
-    // [skipped; is constant] unsigned int (7)version = 1;
-
-    uint8_t seqProfile;           // unsigned int (3) seq_profile;
-    uint8_t seqLevelIdx0;         // unsigned int (5) seq_level_idx_0;
-    uint8_t seqTier0;             // unsigned int (1) seq_tier_0;
-    uint8_t highBitdepth;         // unsigned int (1) high_bitdepth;
-    uint8_t twelveBit;            // unsigned int (1) twelve_bit;
-    uint8_t monochrome;           // unsigned int (1) monochrome;
-    uint8_t chromaSubsamplingX;   // unsigned int (1) chroma_subsampling_x;
-    uint8_t chromaSubsamplingY;   // unsigned int (1) chroma_subsampling_y;
-    uint8_t chromaSamplePosition; // unsigned int (2) chroma_sample_position;
-
-    // unsigned int (3)reserved = 0;
-    // unsigned int (1)initial_presentation_delay_present;
-    // if (initial_presentation_delay_present) {
-    //     unsigned int (4)initial_presentation_delay_minus_one;
-    // } else {
-    //     unsigned int (4)reserved = 0;
-    // }
-} avifCodecConfigurationBox;
-
 // ---------------------------------------------------------------------------
 // avifIO
 
diff --git a/include/avif/internal.h b/include/avif/internal.h
index b854698..3be3908 100644
--- a/include/avif/internal.h
+++ b/include/avif/internal.h
@@ -397,6 +397,30 @@
 // This is to make it clear that the box size is currently unknown, and will be determined later (with a call to avifRWStreamFinishBox)
 #define AVIF_BOX_SIZE_TBD 0
 
+typedef struct avifCodecConfigurationBox
+{
+    // [skipped; is constant] unsigned int (1)marker = 1;
+    // [skipped; is constant] unsigned int (7)version = 1;
+
+    uint8_t seqProfile;           // unsigned int (3) seq_profile;
+    uint8_t seqLevelIdx0;         // unsigned int (5) seq_level_idx_0;
+    uint8_t seqTier0;             // unsigned int (1) seq_tier_0;
+    uint8_t highBitdepth;         // unsigned int (1) high_bitdepth;
+    uint8_t twelveBit;            // unsigned int (1) twelve_bit;
+    uint8_t monochrome;           // unsigned int (1) monochrome;
+    uint8_t chromaSubsamplingX;   // unsigned int (1) chroma_subsampling_x;
+    uint8_t chromaSubsamplingY;   // unsigned int (1) chroma_subsampling_y;
+    uint8_t chromaSamplePosition; // unsigned int (2) chroma_sample_position;
+
+    // unsigned int (3)reserved = 0;
+    // unsigned int (1)initial_presentation_delay_present;
+    // if (initial_presentation_delay_present) {
+    //     unsigned int (4)initial_presentation_delay_minus_one;
+    // } else {
+    //     unsigned int (4)reserved = 0;
+    // }
+} avifCodecConfigurationBox;
+
 typedef struct avifSequenceHeader
 {
     uint32_t maxWidth;