Make: conditionally build the event_vhost library

This library was being consistently built, even when the CONFIG_VHOST
directory was set to false. On the shared object build, this leads to a
few undefined references from the event_vhost.so library when trying to
link against an SPDK application.

This library should be built based on the same conditions as the vhost
library. This assumption is already baked into other aspects of the
build system. See for example the makefile under app/spdk_tgt.

Change-Id: I1c6e651b29785b48455a3a0ce7faaed4319148d9
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/463941


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Broadcom SPDK FC-NVMe CI <spdk-ci.pdl@broadcom.com>