backend-x11: need libdrm headers in build

9ddb3bc315 started using drm_fourcc.h but forgot
to add libdrm headers to the dependencies.

This fixes the build for build-native-meson-no-gl-renderer when a future patch
reduces the dependencies pulled in by the libweston dependency object.

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

libweston/backend-x11/meson.build

@@ -17,6 +17,7 @@ endif
 
 deps_x11 = [
 	dep_libweston_private,
+	dep_libdrm_headers,
 	dep_x11_xcb,
 	dep_lib_cairo_shared,
 	dep_pixman,