build: Prefer gpgrt-config when available.
* src/ksba.m4: Overriding the decision by --with-libksba-prefix, use gpgrt-config ksba when gpgrt-config is available.
This may offer better migration.
- GnuPG-bug-id: T5034
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>