cbrt_tablegen: Include libm.h

Needed for cbrtf fallback on systems which lack it.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/cbrt_tablegen.c

@@ -21,6 +21,7 @@
  */
 
 #include <stdlib.h>
+#include "libavutil/libm.h"
 #define CONFIG_HARDCODED_TABLES 0
 #include "cbrt_tablegen.h"
 #include "tableprint.h"