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_init.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_WIN32)
+
 #include <stdlib.h>
 
 static const GUID _glfw_GUID_DEVINTERFACE_HID =
@@ -725,3 +727,5 @@ void _glfwTerminateWin32(void)
     freeLibraries();
 }
 
+#endif // _GLFW_WIN32
+