gnu: dnsmasq: Enable dbus support.

* gnu/pacakges/dns.scm (dnsmasq): Enable dbus support to allow for
NetworkManager to use dnsmasq.
[native-inputs]: Add pkg-config.
[inputs]: Add dbus.
[arguments]: Add COPTS="-DHAVE_DBUS" to make-flags.

Signed-off-by: 宋文武 <iyzsong@member.fsf.org>