build: Make sure GTK compiles without Vulkan headers installed

Previously, code would work fine with --disable-vulkan if the Vulkan
headers were installed - code would happily just use them as they're
installed in /usr/include.

gdk/gdkvulkancontext.c

@@ -20,11 +20,9 @@
 
 #include "config.h"
 
-#include "gdkvulkancontextprivate.h"
+#include "gdkvulkancontext.h"
 
-#ifdef GDK_RENDERING_VULKAN
-#include <vulkan/vulkan.h>
-#endif
+#include "gdkvulkancontextprivate.h"
 
 #include "gdkdisplayprivate.h"
 #include "gdkinternals.h"
@@ -33,6 +31,7 @@
 typedef struct _GdkVulkanContextPrivate GdkVulkanContextPrivate;
 
 struct _GdkVulkanContextPrivate {
+#ifdef GDK_RENDERING_VULKAN
   VkSurfaceKHR surface;
   VkSurfaceFormatKHR image_format;
 
@@ -42,8 +41,9 @@ struct _GdkVulkanContextPrivate {
 
   guint n_images;
   VkImage *images;
+#endif
 
-  uint32_t draw_index;
+  guint32 draw_index;
 };
 
 enum {

gdk/gdkvulkancontextprivate.h

@@ -47,8 +47,10 @@ struct _GdkVulkanContextClass
 {
   GdkDrawContextClass parent_class;
 
+#ifdef GDK_RENDERING_VULKAN
   VkResult     (* create_surface)       (GdkVulkanContext       *context,
                                          VkSurfaceKHR           *surface);
+#endif
 };
 
 #ifdef GDK_RENDERING_VULKAN