build: Provide gpgme-glib.pc too.

* src/gpgme-glib.pc.in: New.
* src/gpgme.pc.in (avail_lang): Remove.

Provide gpgme-glib.pc for gpgme-glib library. The avial_lang
information is no use because *.pc is for C.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>