Add null joystick backend

This prepares the X11 backend to support other joystick APIs, for
example the FreeBSD libusb one.

src/linux_joystick.h

@@ -45,11 +45,9 @@ typedef struct _GLFWjoystickLinux
 //
 typedef struct _GLFWlibraryLinux
 {
-#if defined(__linux__)
     int             inotify;
     int             watch;
     regex_t         regex;
-#endif /*__linux__*/
 } _GLFWlibraryLinux;