chan_mgcp: Enforce locking for oseq

This restricts direct usage of global oseq so that all accesses are
locked and threads are not racing to get oseq values that they did not
claim.

This also fixes a build error in res_pktccops under dev mode.

(closes issue ASTERISK-23100)
Reported by: adomjan
Patch by: adomjan
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Merged revisions 406037 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 406038 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 406049 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@406078 65c4cc65-6c06-0410-ace0-fbb531ad65f3