Add a FreeMonitor platform API

This fixes the leak of a wl_output object everytime a monitor is freed.

src/internal.h

@@ -607,6 +607,7 @@ void _glfwPlatformSetCursor(_GLFWwindow* window, _GLFWcursor* cursor);
 const char* _glfwPlatformGetScancodeName(int scancode);
 int _glfwPlatformGetKeyScancode(int key);
 
+void _glfwPlatformFreeMonitor(_GLFWmonitor* monitor);
 void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos);
 void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
                                          float* xscale, float* yscale);