Fixed build error (thanks @sezero!)

(cherry picked from commit ac5a61cd608cea3118fe581a9f8fffe67da29397)
(cherry picked from commit 67c60e0b9ca870c91b5b690361bad0329dfe8a10)

src/joystick/linux/SDL_sysjoystick.c

@@ -907,7 +907,9 @@ static void LINUX_ScanSteamVirtualGamepads(void)
     int num_virtual_gamepads = 0;
     int virtual_gamepad_slot;
     VirtualGamepadEntry *virtual_gamepads = NULL;
+#ifdef SDL_USE_LIBUDEV
     int class;
+#endif
 
     count = scandir("/dev/input", &entries, filter_entries, NULL);
     for (i = 0; i < count; ++i) {