Another case of missing wayland-util.h, as we didn't pass any libwayland
CFLAGS. This is triggerable on a system, where libwayland is installed
in a custom prefix, and pixman, cairo, libpng, and webp are either
not installed or are installed in the standard path.

COMPOSITOR_CFLAGS contains more than just the libwayland CFLAGS, though.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>