vhost, unit: fix DPDK include issue in Makefile.

According to the env.mk, DPDK include file can be
passed by user, so we need to use DPDK_INC to
avoid the compilation issue.

Change-Id: I24f6ca496eefe16c18bba7278cc6d00120da79e1
Signed-off-by: Ziye Yang <optimistyzy@gmail.com>
Reviewed-on: https://review.gerrithub.io/373235


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>