gpg-error.m4: Fix use with older gpgrt-config (< 1.33).

* src/gpg-error.m4: When gpgrt-config doesn't work expected for
gpg-error.pc, unset GPGRT_CONFIG.

Found the issue in the libgcrypt build with libgpg-error 1.31.

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