zebra: Allow zebra_mlag to compile with no j factor

zebra: Allow zebra_mlag to compile with no j factor

If you compile FRR with no j factor zebra_mlag.c fails to
build because the vtysh extraction methodology runs first
before the protobuf compiler runs and that compilation does
not have the proper dependancy chain built for the inclusions
that zebra_mlag.c had.  Moving the DEF* code into a zebra_mlag_vty.c
which can be included in the vtysh extraction code and has
no mlag.proto dependancies makes the compilation work better.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>