mpv: Fix X11 support by adding libXrandr

This is a follow-up to 6b404b94

, which
fixed the build after SDL no longer leaks through X libraries.

However the commit didn't actually pass all the required dependencies to
mpv, because the configure phase of mpv still disabled X support:

  Checking for X11
  err: Package xrandr was not found in the pkg-config search path.
  Perhaps you should add the directory containing `xrandr.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'xrandr' found

So by adding libXrandr, we finally have support for X again.

Trivia: While I'm usually very pesky about line lengths this is one of
the very rare occasions where I didn't break the lines after 79
characters because it would have messed up the overall style of the
file.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @AndersonTorres, @fuuzetsu, @fpletz, @orivej