gpgrt-config: It's distributed script.

* src/Makefile.am (dist_bin_SCRIPTS): Add gpgrt-config.
(bin_SCRIPTS): Remove gpgrt-config.

This is the automake thing; Since gpgrt-config is not generated
script, we should use dist_bin_SCRIPTS here.

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