Build fix:

This is the second attempt to fix the build problem that some people
have seen (I have received inconsistent reports).  This should
force chan_mgcp to build on systems where it can.  It was tested
on NetBSD 5.0, thus ensuring that it doesn't break previously
working systems; and NetBSD 6.99.7, where I finally saw the problem
that some people were reporting.

(jnemeth)