Fix libdrm includes

This removes any assumptions about how the libdrm headers are installed,
and uses the pkg-config include directories as we're "supposed to".
This only adds a partial dependency, since we don't actually need to
link against libdrm.

examples/meson.build

@@ -80,6 +80,7 @@ examples = {
 			libavcodec,
 			libavformat,
 			libavutil,
+			drm.partial_dependency(includes: true), # <drm_fourcc.h>
 			threads,
 			wayland_client,
 			wlr_protos,