core: Rename the gpgrt_log_levels enum values.

* src/gpg-error.h.in (gprt_log_levels): Rename to GPGRT_LOGLVL.

Explicitly naming them and not distinguishing them from the other log
constants is a Good Thing anyway. It also helps against the symbol name
clash with the GPGRT enum values from gnupg's logging module.

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