onig: do not use system headers

When build on host with older eglibc (Ubuntu 12.04) build fails with:

/tmp/OE/build/tmp-eglibc/sysroots/genericarmv8/usr/include/bits/predefs.h:23:3: error: #error "Never use <bits/predefs.h> directly; include <stdc-predef.h> instead."

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>