avcodec/cabac_functions: Add missing headers

Fixes make checkheaders on PPC, for which no arch-specific header
exists that indirectly includes attributes.h.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/cabac_functions.h

@@ -30,6 +30,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "libavutil/attributes.h"
+#include "libavutil/intmath.h"
 #include "cabac.h"
 #include "config.h"