mplayer: Fix HEAD build by requiring X11

The HEAD formula won't build without X11 (it can't find libgl), so I force-enabled the "with-x" option for HEAD builds.  mplayer doesn't seem to use X11 even without the --disable-x11 configure option, anyways.

I get a warning on install but it works fine:

 	==> Finishing up
	ln -s ../Cellar/mplayer/HEAD/bin/mplayer mplayer
	ln -s ../Cellar/mplayer/HEAD/bin/mencoder mencoder
	ln -s ../../../Cellar/mplayer/HEAD/share/man/man1/mplayer.1 mplayer.1
	ln -s ../../../Cellar/mplayer/HEAD/share/man/man1/mencoder.1 mencoder.1
	ln -s ../../Cellar/mplayer/HEAD mplayer
	ln -s ../Cellar/mplayer/HEAD mplayer
	Warning: Could not fix install names for /usr/local/Cellar/mplayer/HEAD/bin/mplayer