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/win32_monitor.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_WIN32)
+
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>
@@ -545,3 +547,5 @@ GLFWAPI const char* glfwGetWin32Monitor(GLFWmonitor* handle)
     return monitor->win32.publicDisplayName;
 }
 
+#endif // _GLFW_WIN32
+