Make sure libipvs.a is built before ipvsadm

There is no explicit rule in the top-level Makefile to build libipvs.a.  It
is built by the phony target `libs`.  However, there is no guarantee of the
order in which the prerequisites of the `all` target are built, so make may
attempt to link ipvsadm to libipvs.a before it has finished building
libipvs.a.

Add a rule to express the dependency of `$(STATIC_LIBS)` on `libs`.

Signed-off-by: Jeremy Sowden <azazel@debian.org>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@kernel.org>