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.
diff --git a/include/SDL_config_iphoneos.h b/include/SDL_config_iphoneos.h
index a27260e..94ab693 100644
--- a/include/SDL_config_iphoneos.h
+++ b/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
