gsk: Include the private header

Both gskprivate.c and gskvulkanrenderer.c call `gsk_ensure_resources`
but fail in clude `gskprivate.h`, where the function is declared.

gsk/gskprivate.c

@@ -1,4 +1,5 @@
 #include "gskresources.h"
+#include "gskprivate.h"
 
 static gpointer
 register_resources (gpointer data)

gsk/gskvulkanrender.c

@@ -1,5 +1,7 @@
 #include "config.h"
 
+#include "gskprivate.h"
+
 #include "gskvulkanrenderprivate.h"
 
 #include "gskrendererprivate.h"