Don't build Vulkan video backend on macOS

There's no official implementation of the Vulkan API,
and Dolphin currently isn't set-up to work with the
single, commercially-available third-party implementation.

Source/Core/Core/CMakeLists.txt

@@ -236,7 +236,6 @@ set(LIBS
 	sfml-network
 	sfml-system
 	videonull
-	videovulkan
 	videoogl
 	videosoftware
 	z
@@ -249,6 +248,10 @@ if(LIBUSB_FOUND)
 	        	       	IPC_HLE/WII_IPC_HLE_Device_usb_bt_real.cpp)
 endif(LIBUSB_FOUND)
 
+if(NOT APPLE)
+	set(LIBS ${LIBS} videovulkan)
+endif()
+
 set(LIBS ${LIBS} ${MBEDTLS_LIBRARIES})
 
 if(WIN32)