xkbcommon.h build fix for missing stdarg.h
In file included from external/collabora/libxkbcommon/src/xkb-priv.h:87,
from external/collabora/libxkbcommon/src/xkbcomp/xkbcomp.h:30,
from external/collabora/libxkbcommon/src/xkbcomp/xkbcomp-priv.h:30,
from external/collabora/libxkbcommon/src/xkbcomp/action.h:30,
from external/collabora/libxkbcommon/src/xkbcomp/action.c:27:
external/collabora/libxkbcommon/xkbcommon/xkbcommon.h:279: error: expected declaration specifiers or '...' before 'va_list'
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>