Fix FindWaylandProtocols for cross-compilation mode

Fix WaylandProtocols_DATADIR returned value, if PKG_CONFIG_SYSROOT_DIR environment variable is defined, which is usually the case in cross-compilation mode.

Without that FindWaylandProtocols returns a raw value taken from the .pc config file which usually starts with the: /usr prefix and which later on causes the wayland-scanner to start looking for wayland-protocol file from the host machine instead of using that one from the sysroot.

Signed-off-by: Damian Wrobel dwrobel@ertelnet.rybnik.pl