configure.ac: unconditionally check for expat

configure.ac: unconditionally check for expat

Earlier commits moved the xmlconfig library to a wider userbase.
Thus having the check within --enable-dri is insufficient.

Upon closer look, nine needed it from it's early days - 948e6c52282
("nine: Add drirc options (v2)")

Fixes: 601093f95ddf ("xmlconfig: move into src/util")
Cc: Axel Davy <axel.davy@ens.fr>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (IRC)