lib/util: fix call to dbghdrclass() for DEBUGC()

lib/util: fix call to dbghdrclass() for DEBUGC()

dbghdrclass() sets the global 'current_msg_class' and for that
DEBUGC() should pass the given dbgc_class instead of the per file
DBGC_CLASS.

This is important with the new per class logfile with:

 log level = 1 dsdb_audit:10@/var/log/samba/log.dsdb_audit

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13915

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
(similar to commit bb0ffbf38cb1955c9e400003add680eabcf706a6)