xorg-lib-common: Add variable to set tarball type

xorg-lib-common: Add variable to set tarball type

Upstream has switched some new releases from bz2 to xz compression. Add
an XORG_EXT variable so recipes can set the file name extension needed
for the compression type.

Following the approach in oe-core/master:
  6a8068e036b4b2a40b38896275b936916b4db76e  xorg-lib-common: Add variable to set tarball type
use a variable for the tarball suffix/compression format.

Signed-off-by: Robert Joslyn <robert.joslyn@redrectangle.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Yogita Urade <yogita.urade@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>