Revert "cbrt_tablegen: Include libm.h"

This code runs on the host and should not use libm.h, which is meant
for the target.

This reverts commit 2768b71798.

libavcodec/cbrt_tablegen.c

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