perf color: Add missing stdarg.g to color.h

It was getting the va_list definition by luck.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-4mavb7pgt2nw9lsew1xuez09@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>