configure.ac: add missing llvm dependencies to .pc files

v2: Only add as dependencies for gallium-osmesa and gallium-xlib

CC: <mesa-stable@lists.freedesktop.org>
Signed-of-by: Chuck Atkins <chuck.atkins@kitware.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 6ac5e851)