fixed fullscreen issue

in my OSD.xml was

<osd enabled="yes" type="button" x="-60" y="220" command="gui.fullscreen()" src="gui_fullscreen.png"/>

i changed in

<osd enabled="yes" type="button" x="-60" y="220" command="gui.fullscreen=!gui.fullscreen" src="gui_fullscreen.png"/>

and the button is now working, also i can see the fullscreen option in menu