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.

render/meson.build

@@ -22,7 +22,13 @@ lib_wlr_render = static_library(
 	),
 	glapi,
 	include_directories: wlr_inc,
-	dependencies: [egl, glesv2, pixman, wayland_server],
+	dependencies: [
+		egl,
+		drm.partial_dependency(includes: true), # <drm_fourcc.h>
+		glesv2,
+		pixman,
+		wayland_server
+	],
 )
 
 wlr_render = declare_dependency(