loader: rework xmlconfig dependency

Currently xmlconfig is conditionally used, only when --enable-dri is
available.

As the library has moved to src/util and has wider wisebase, this guard
is no longer correct. Strictly speaking - it wasn't since the
introduction of xmlconfig into st/nine a while ago.

Unconditionally enable xmlconfig and drop the linking. As said before
there's other users of the library, so depending on the configure
options we will get multiple definitions of said symbols.

NOTE: To avoid breaking other combinations, this commit adds the
xmlconfig link to the required places - throughout gallium and the DRI
loaders.

Cc: Aaron Watry <awatry@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>