Add gamepad mapping filtering by backend

This fixes gamepad mappings being included in GLFW binaries that would
then never use them because they were for a different platform.

src/win32_joystick.h

@@ -28,6 +28,7 @@
 #define _GLFW_PLATFORM_LIBRARY_JOYSTICK_STATE struct { int dummyLibraryJoystick; }
 
 #define _GLFW_PLATFORM_MAPPING_NAME "Windows"
+#define GLFW_BUILD_WIN32_MAPPINGS
 
 // Joystick element (axis, button or slider)
 //