Add conditional compilation for platform units

This is a step towards being able to compile GLFW manually without
needing to duplicate a lot of platform- or OS-specific logic.

src/wl_monitor.c

@@ -28,6 +28,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_WAYLAND)
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -270,3 +272,5 @@ GLFWAPI struct wl_output* glfwGetWaylandMonitor(GLFWmonitor* handle)
     return monitor->wl.output;
 }
 
+#endif // _GLFW_WAYLAND
+