cmdutils: fix expected signature for show_colors() function

Fix warning:
In file included from ffprobe.c:2557:0:
cmdutils_common_opts.h:17:5: warning: initialization from incompatible pointer type [enabled by default]
cmdutils_common_opts.h:17:5: warning: (near initialization for ‘real_options[16].u.func_arg’) [enabled by default]

Spotted-by: Paul B Mahol

cmdutils.h

@@ -492,7 +492,7 @@ int show_sample_fmts(void *optctx, const char *opt, const char *arg);
  * Print a listing containing all the color names and values recognized
  * by the program.
  */
-void show_colors(void *optctx, const char *opt, const char *arg);
+int show_colors(void *optctx, const char *opt, const char *arg);
 
 /**
  * Return a positive value if a line read from standard input