Remove unnecessary "lmgr.h" in stat_utils.c

Hi hackers,

While reviewing the import/export statistics, I noticed that relation 
locking are handled via relation_open() and relation_close() in 
stats_lock_check_privileges(), and no calls to other lock-manager 
routines are actually used there. As a result, the inclusion of the 
lock-manager header

#include "storage/lmgr.h"

is not needed. I have attached a small patch which simply removes that 
include.

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Best regards,
Ilia Evdokimov,
Tantor Labs LLC.