gitlab-ci: cache meson subprojects

This saves another 30sec here and reducing clone operations on the
gnome gitlab instance is probably a good idea as well.

.gitlab-ci.yml

@@ -11,6 +11,11 @@ before_script:
 cache:
   paths:
     - _ccache/
+    - subprojects/gdk-pixbuf/
+    - subprojects/glib/
+    - subprojects/graphene/
+    - subprojects/libepoxy/
+    - subprojects/pango/
 
 build:
   stage: build