gitlab: don't run the DCO check on git master
gitlab: don't run the DCO check on git master

The "only: branches" does not in fact exclude "master", so it must be
excluded explicitly.

Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

nertpinx/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jfehlig/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,jardasgit/libvirt,zippy2/libvirt,jardasgit/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt