clang compiler warnings: Ignore -Wunused-command-line-argument

Asterisk's build system has a tendency to pass include directives for libraries
to everything compiled within a particular group of source files. This means
we pass the header for libxml2 to things that don't necessarily need it. As a
result, we ignore this particular warning.

Review: https://reviewboard.asterisk.org/r/4545/

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4545.patch submitted by dkdegroot (License 6600)
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Merged revisions 433720 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 433721 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433722 65c4cc65-6c06-0410-ace0-fbb531ad65f3