MFH: r482753

databases/xtrabackup: fix build due to stricter llvm60 c++

PR: 230836
Submitted by: Sergey Akhmatov <sergey@akhmatov.ru>
Reported by: rainer@ultra-secure.de
Reviewed by: ddrinnon@cdor.net,
Approved by: aleks@twindb.com (maintainer timeout)
Obtained from: https://github.com/percona/percona-xtrabackup/commit/09069e9ee8f84df0ba3091f591b9269842e71503#diff-765605c50b26c5ee91da8227ac1093b2
Approved by: portmgr (unbreak blanket approval)