gpg-error-config: Fix the place of *.pc (for multilib).

* src/Makefile.am (pkgconfigdir): It should be under libdir.
* src/gpg-error-config-new.in: Likewise.

• Reported-by: Alon Bar-Lev <alon.barlev@gmail.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>