Fix compilation issues for OpenBSD

This patch addresses compilation issues for OpenBSD. Specifically, it
addresses:
 * It allows including <sys/vmmeter.h> in asterisk.c
 * Provides a needed (size_t) cast in xmldoc.c

In 13+, it also addresses a conditional inclusion in loader.c.

Review: https://reviewboard.asterisk.org/r/4506

ASTERISK-24880 #close
Reported by: snuffy
Tested by: snuffy
patches:
  misc-openbsd.diff uploaded by snuffy (License 5024)
........

Merged revisions 433245 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@433247 65c4cc65-6c06-0410-ace0-fbb531ad65f3