vulkan: Include a copy of vulkan.h and vk_platform.h.

Now we can provide Vulkan support in the build even if the build box doesn't
have a Vulkan SDK, since we dynamically link to the library anyhow.

src/video/SDL_vulkan_internal.h

@@ -59,7 +59,7 @@
  * inclusion of vulkan has the appropriate USE_PLATFORM defined, hence
  * the above. */
 #define VK_NO_PROTOTYPES
-#include "vulkan/vulkan.h"
+#include "./khronos/vulkan/vulkan.h"
 
 extern const char *SDL_Vulkan_GetResultString(VkResult result);