Fix function signature in null joystick backend

Related to #1043.

src/null_joystick.c

@@ -31,7 +31,7 @@
 //////                       GLFW platform API                      //////
 //////////////////////////////////////////////////////////////////////////
 
-int _glfwPlatformPollJoystick(int jid, int mode)
+int _glfwPlatformPollJoystick(_GLFWjoystick* js, int mode)
 {
     return GLFW_FALSE;
 }