cc-tool: fix missing stdarg.h include on uClibc

This commit adds a patch to fix a missing <stdarg.h> include for
va_list.

Fixes:

  http://autobuild.buildroot.org/results/9c8/9c828b1da8983577f93bdfae5275fc5122ae874f/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>