build: Do not define logging.h constants for libgpg-error dev versions.

* common/logging.h [GPGRT_LOG_WITH_PREFIX]: Do not define the log
constants.

logging.h uses constants we plan to use for future versions of
libgpg-error. My dev version already has the logging functions and
thus I run into a conflict. This patch protects against this and make
the GnuPG work with later libgpg-error versions.

It was not the best idea to use constants from a planned libgpg-error
in the first place. The actual problem are the enums, the macros
won't harm.

• Signed-off-by: Werner Koch <wk@gnupg.org>