CMake: Include X11 Xlib header path

Build fails trying to find X11/Xlib.h via ext/vulkan/vulkan.h.

CMakeLists.txt

@@ -174,6 +174,8 @@ option(USE_UBSAN "Use undefined behaviour sanitizer" OFF)
 if(UNIX AND NOT (APPLE OR ANDROID) AND VULKAN)
 	if(USING_X11_VULKAN)
 		message("Using X11 for Vulkan")
+		find_package(X11)
+		include_directories(${X11_Xlib_INCLUDE_PATH})
 		add_definitions(-DVK_USE_PLATFORM_XLIB_KHR)
 	else()
 		message("NOT using X11 for Vulkan")