radeon/llvm: Link against libgallium.la to fix an undefined symbol

radeon/llvm: Link against libgallium.la to fix an undefined symbol

Ported from downstream:
http://anonscm.debian.org/gitweb/?p=pkg-xorg/lib/mesa.git;a=blob;f=debian/patches/119-libllvmradeon-link.patch;h=ee47f8a07dbf33c32f8b57faed923680ed6648fb;hb=refs/heads/ubuntu%2B1

Fixes a regression introduced with
f70c3853513637fa6ed38e75f73d472a9fa61213

NOTE: This is a candidate for the 9.1 branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62434
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>