Add missing locales to configure.ac

Commits 40761898692e5063957bfa2518cca3d35b2e354a and
f198c6aa98f88ff285d903175a3c4c0fd33a4575 added two new locales
(sr_CS.UTF-8 and km_KH.UTF-8), but didn't list them in configure.ac,
meaning they're not included in tarballs.

Signed-off-by: Julien Cristau <jcristau@debian.org>
Reviewed-by: James Cloos <cloos@jhcloos.com>