mplayer: use UTF-8 charset to avoid iconv dependency

The mplayer configure script was failing with:

  Checking for iconv program ... no
  No working iconv program found, use
  --charset=UTF-8 to continue anyway.
  If you also have problems with iconv library functions use --charset=noconv.
  Messages in the GTK-2 interface will be broken then.

Therefore, we pass --charset=UTF-8 instead of --charset=US-ASCII.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>