xserver-xorg_1.10.1.bb: Replace use of --variable=sdkdir with --cflags-only-I

This fixes the build failures of xf86-video-fbdev on build systems which
have xorg-xserver development packages installed.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>