iOS: Fix compiling using the iOS 7 SDK, partially broken since MoltenVK support was added.

Note that apps submitted to the iOS App Store *must* use a modern iOS SDK (currently iOS 10 is probably the minimum), however the SDK used to build is separate from the minimum iOS version an app supports at runtime.

include/SDL_config_iphoneos.h

@@ -140,8 +140,8 @@
 #define SDL_VIDEO_RENDER_OGL_ES 1
 #define SDL_VIDEO_RENDER_OGL_ES2    1
 
-/* Enable Vulkan support */
-#if !TARGET_OS_SIMULATOR && !TARGET_CPU_ARM // Only 64-bit devices have Metal
+/* Enable Vulkan support on 64-bit devices when an iOS 8+ SDK is used. */
+#if !TARGET_OS_SIMULATOR && !TARGET_CPU_ARM && defined(__IPHONE_8_0)
 #define SDL_VIDEO_VULKAN 1
 #else
 #define SDL_VIDEO_VULKAN 0