build_tools/mkpkgconfig: Fix Cflags concatenation error in asterisk.pc

The mkpkgconfig script incorrectly concatenates Cflags options together. As an
example, the following:
Cflags: -I/usr/include/libxml2 -g3

Is instead generated as:
Cflags: -I/usr/include/libxml2-g3

This patch corrects the generation of Cflags in mkpkgconfig such that the
Cflags options are output correctly.

Review: https://reviewboard.asterisk.org/r/3707/

ASTERISK-23991 #close
Reported by: Diederik de Groot
patches:
  fix_mkpkgconfig.diff uploaded by Diederik de Groot (License 6600)
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Merged revisions 430589 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 430590 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430591 65c4cc65-6c06-0410-ace0-fbb531ad65f3