Makefile: Disable XML doc validation

make_xml_documentation was being called with the --validate
flag set when it shouldn't have been.  This was causing
build failures if neither xmllint nor xmlstarlet were installed.
The correct behavior is to simply print a message that either
one of those tools should be installed for validation and
continue with the build.

ASTERISK-29988

Change-Id: Idc6c44114e7dd3fadae183a4e22f4fdba0b8a645