xserver_xorg-server: fix libunwind on MIPS with 32-bit pointers

Adding a patch to fix a compilation problem of Xorg + libunwind support
when building it on MIPS variants with 32-bit pointers.

Related:

  http://lists.busybox.net/pipermail/buildroot/2014-December/114404.html

Patch submitted upstream:

  http://lists.x.org/archives/xorg-devel/2015-January/045226.html

Patch reviewed by an Xorg developer:

  http://lists.x.org/archives/xorg-devel/2015-January/045383.html

Fixes:

  http://autobuild.buildroot.net/results/87b/87be2c95957f9925c1258812e536df72689fc5da/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>