example/interrupt: do not compile interrupt_tgt without vhost

The interrupt_tgt depends on vhost lib, so it should not be
compiled when SPDK is not using it.

This resolves compilation errors with just:
./configure --without-vhost

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: Iad2dff6bb73e1b3d389b98f0dfbf157e576797be
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/5340


Community-CI: Broadcom CI
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>