spec: Drop BuildRequires on dnsmasq

spec: Drop BuildRequires on dnsmasq

It's been an optional build time dependency for a long time, so
if Meson couldn't find it the only consequence was that libvirt
would look for it at runtime instead, which is what we are doing
for most of our non-library dependencies anyway.

Since 5c98d1cee0be we've stopped even looking for it at build
time, so there's no point in having it installed in the build
environment.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>