wayland: always build wayland-scanner

wayland: always build wayland-scanner

By passing --disable-scanner to use a native wayland-scanner binary a target
wayland-scanner binary isn't built, which is a problem if you want to use it on
the target or in a SDK.

Instead, always build a target wayland-scanner binary, and have an option to
control whether that binary or a host-provided binary is used at build time.

[ YOCTO #7931 ]

Signed-off-by: Ross Burton <ross.burton@intel.com>