pgsql: Only log 'process acquired lock' if we actually did get the lock.

Log Message:
-----------
Only log 'process acquired lock' if we actually did get the lock. This
test seems inessential right now since the only control path for not
getting the lock is via CHECK_FOR_INTERRUPTS which won't return control
to ProcSleep, but it would be important if we ever allow the deadlock
code to kill someone else's transaction instead of our own.

Modified Files:
--------------
pgsql/src/backend/storage/lmgr:
proc.c (r1.189 -> r1.190)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c.diff?r1=1.189&r2=1.190)