shared: fix build of libshared-cairo

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>

shared/Makefile.am

@@ -11,6 +11,7 @@ libshared_la_SOURCES =				\
 
 libshared_cairo_la_CFLAGS =			\
 	$(GCC_CFLAGS)				\
+	$(COMPOSITOR_CFLAGS)			\
 	$(PIXMAN_CFLAGS)			\
 	$(CAIRO_CFLAGS)				\
 	$(PNG_CFLAGS)				\