Move STAT_TYPE enum to source file.

In the header, all we need is number of stat types, not the names for actual
types.

Removing it avoids names like 'Y', 'U', 'V' and 'ALL' being visible
in all files that include the encoder.h header.

Change-Id: I874a73a3cfe6bcb29aedea102077a52addc49af6

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 82b75dc..be75871 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -100,6 +100,10 @@
 FILE *keyfile;
 #endif
 
+#if CONFIG_INTERNAL_STATS
+typedef enum { Y, U, V, ALL } STAT_TYPE;
+#endif  // CONFIG_INTERNAL_STATS
+
 static INLINE void Scale2Ratio(AOM_SCALING mode, int *hr, int *hs) {
   switch (mode) {
     case NORMAL:

av1/encoder/encoder.h

diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index 8d67c67..20eefe3 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -307,13 +307,15 @@
   unsigned char *map;
 } ActiveMap;
 
-typedef enum { Y, U, V, ALL } STAT_TYPE;
+#define NUM_STAT_TYPES 4  // types of stats: Y, U, V and ALL
 
 typedef struct IMAGE_STAT {
-  double stat[ALL + 1];
+  double stat[NUM_STAT_TYPES];
   double worst;
 } ImageStat;
 
+#undef NUM_STAT_TYPES
+
 typedef struct {
   int ref_count;
   YV12_BUFFER_CONFIG buf;