Fix build --without-network

Fix build --without-network

In order not to bring in any link dependencies, bridge driver doesn't
use the usual stubs as other conditionally-built code does.  However,
having the function as a macro imposes a problem with possibly unused
variables if just defined as "0".  This was worked around by using
(dom=dom, iface=iface, 0) which should act like a 0 if used in a
condition.  However, gcc still bugs about that, so I came up with
another way how to fix that.

Using static inline functions in the header won't collide with anything,
it fixes the bug and does one thing that the macro didn't do.  It checks
whenther passed variables are pointers of compatible type.  It has only
one downside, and that is that we need to either a) define it with
ATTRIBUTE_UNUSED, which needs an exception in cfg.mk or b) do something
like ignore_value(variable); in the function body.  I went with the
first variant.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>