mesa3d: add patch to fix wayland-scanner issues

mesa3d: add patch to fix wayland-scanner issues

mesa3d tries to use the pkg-config specified wayland-scanner when
wayland is enabled, which points to /usr/bin/wayland-scanner which is
incorrect.
The correct fix is trying to use the PATH wayland-scanner first if it's
available rather than the pkg-config specified one, since tweaking a
staging .pc file with a host binary is wrong. Fixes:
http://autobuild.buildroot.net/results/c0c/c0ca3778c390a33c27073553ca3bef7ac7c5784b/

Patch sent upstream:
https://lists.freedesktop.org/archives/mesa-dev/2016-November/135665.html

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>