gpu: Fix forward declaration and inclusion of PrivateGPUDriver

src/gpu/SDL_gpu.c

@@ -158,6 +158,9 @@
 
 #ifndef SDL_GPU_DISABLED
 static const SDL_GPUBootstrap *backends[] = {
+#ifdef SDL_GPU_PRIVATE
+    &PrivateGPUDriver,
+#endif
 #ifdef SDL_GPU_METAL
     &MetalDriver,
 #endif

src/gpu/SDL_sysgpu.h

@@ -989,7 +989,7 @@ extern "C" {
 extern SDL_GPUBootstrap VulkanDriver;
 extern SDL_GPUBootstrap D3D12Driver;
 extern SDL_GPUBootstrap MetalDriver;
-extern SDL_GPUBootstrap PS5Driver;
+extern SDL_GPUBootstrap PrivateGPUDriver;
 
 #ifdef __cplusplus
 }