List AVIF_COLOR_PRIMARIES_SRGB after BT709

This order suggests that AVIF_COLOR_PRIMARIES_BT709 is the main name of
the constant and AVIF_COLOR_PRIMARIES_SRGB is an alias. The association
of color_primaries value 1 with BT.709 is well known.

include/avif/avif.h

@@ -311,8 +311,8 @@ enum
     // This is actually reserved, but libavif uses it as a sentinel value.
     AVIF_COLOR_PRIMARIES_UNKNOWN = 0,
 
-    AVIF_COLOR_PRIMARIES_SRGB = 1,
     AVIF_COLOR_PRIMARIES_BT709 = 1,
+    AVIF_COLOR_PRIMARIES_SRGB = 1,
     AVIF_COLOR_PRIMARIES_IEC61966_2_4 = 1,
     AVIF_COLOR_PRIMARIES_UNSPECIFIED = 2,
     AVIF_COLOR_PRIMARIES_BT470M = 4,