build: simplify check for WITH_MACVTAP

build: simplify check for WITH_MACVTAP

macvtap support was added to the Linux kernel in 2.6.33. libvirt
checked for this by looking for MACVLAN_MODE_BRIDGE and IFLA_VF_MAX in
linux/if_link.h. This hasn't been necessary for a very long time, so
just gate on platform == 'linux' (and be sure to complain if someone
tries to enable it on a non-Linux platform).

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>