Make joystick platform code init on demand

This makes joystick support initialize the first time a joystick
function is called, including those gamepad functions that are layered
on top of joystick functions.

Related to #1284.
Related to #1646.

src/win32_joystick.h

@@ -48,9 +48,6 @@ typedef struct _GLFWjoystickWin32
     GUID                    guid;
 } _GLFWjoystickWin32;
 
-
-void _glfwInitJoysticksWin32(void);
-void _glfwTerminateJoysticksWin32(void);
 void _glfwDetectJoystickConnectionWin32(void);
 void _glfwDetectJoystickDisconnectionWin32(void);