libpcp/lock.c: don't call lockname() unless PM_MULTI_THREAD_DEBUG defined
libpcp/lock.c: don't call lockname() unless PM_MULTI_THREAD_DEBUG defined

Recent diagnostic changes were broken when PM_MULTI_THREAD_DEBUG is not
defined ... we were calling lockname() which is not included in the
library.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp