build-sys: use a separate slirp-obj-y && slirp.mo

build-sys: use a separate slirp-obj-y && slirp.mo

This will allow to have cflags for the whole slirp.mo -objs.
It makes it possible to build tests that links only with
slirp-obj-y (and not the whole common-obj).

It is also a step towards building slirp as a shared library, although
this requires a bit more thoughts to build with
net/slirp.o (CONFIG_SLIRP would need to be 'm') and other build issues.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>