src/io.c: Some cleanup related to fseeko/ftello

Include <unistd.h> for the definition of _POSIX_VERSION. See
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/unistd.h.html

Add the AVIF_ prefix to USE_FSEEKO.

Remove the definition of the unused macro USE_FSEEK_FALLBACK.

src/io.c

diff --git a/src/io.c b/src/io.c
index a27cf38..823be1a 100644
--- a/src/io.c
+++ b/src/io.c
@@ -5,7 +5,7 @@
 // Ensure off_t is 64 bits.
 #undef _FILE_OFFSET_BITS
 #define _FILE_OFFSET_BITS 64
-// Ensure we have some POSIX compatible with fseeko/ftello.
+// Ensure we have some POSIX compatibility with fseeko/ftello.
 #undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
 #endif
@@ -34,22 +34,19 @@
 }
 #else
 
+#include <unistd.h>
+
 #if defined(__ANDROID__)
 #include <android/api-level.h>
 #if __ANDROID_API__ >= 24
-#define USE_FSEEKO
-#else
-#define USE_FSEEK_FALLBACK
+#define AVIF_USE_FSEEKO
 #endif
 #elif defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
 /* Standard Modern POSIX */
-#define USE_FSEEKO
-#else
-/* Unknown or very old platform */
-#define USE_FSEEK_FALLBACK
+#define AVIF_USE_FSEEKO
 #endif
 
-#if defined(USE_FSEEKO)
+#if defined(AVIF_USE_FSEEKO)
 // POSIX large file support
 static_assert(sizeof(off_t) == sizeof(int64_t), "");
 typedef off_t avif_off_t;
@@ -65,6 +62,7 @@
     return ftello(stream);
 }
 #else
+// Unknown or very old platform. Fall back on fseek/ftell.
 typedef long avif_off_t;
 #define AVIF_OFF_MAX LONG_MAX
 
@@ -77,7 +75,7 @@
 {
     return ftell(stream);
 }
-#endif // defined(USE_FSEEKO)
+#endif // defined(AVIF_USE_FSEEKO)
 
 #endif // defined(_WIN32)