xserver_xorg-server: enable libunwind for MIPS with 32-bit pointers

libunwind support was broken for MIPS variants with 32-bit pointers so
we disabled it:

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



But now we have a patch to fix that problem so we can enable it again.

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