redhat: enable dist tarball to build in a chroot

redhat: enable dist tarball to build in a chroot

When building the rpms, we can use a chroot (in my case docker) to
ensure that the BuildRequires are complete.  This test failed with
errors like:

    checking for CARES... no
    configure: error: trying to build nhrpd, but libcares not found. install c-ares and its -dev headers.
    error: Bad exit status from /var/tmp/rpm-tmp.FewvLf (%build)

This is due to a couple missing BuildRequires in the spec file.  Here, we
add those in for all RPM builds.

Testing done:

Ran a docker build on CentOS7 which succeeded.  Loaded the modules onto
CentOS6 to make sure they were at least valid there, that succeeded.

Issue: https://github.com/FRRouting/frr/issues/1930
Signed-off-by: Arthur Jones <arthur.jones@riverbed.com>