metacity: fix X.org dependencies

Metacity is a X client application, so there is no reason to depend on
the X.org server. Instead, depend on the appropriate X libraries.

In addition to the X.org related dependencies, we also add the
host-libxml-parser-perl dependency, otherwise:

  checking for XML::Parser... configure: error: XML::Parser perl module is required for intltool
  make[1]: *** [/home/test/outputs/xorg/build/metacity-2.25.1/.stamp_configured] Error 1
  make: *** [all] Error 2

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>