Mk/Uses: Fix feature checks after my previous {WITH,WITHOUT}_*_PORTS change

Test Plan:
Take cmake.mk for example. Choosing a port with USES=cmake and running
make -V CMAKE_BUILD_TYPE gives "Release" and
make -V CMAKE_BUILD_TYPE WITH_DEBUG=yes gives "Debug". However,
make -V CMAKE_BUILD_TYPE WITH_DEBUG_PORTS=this/port still gives "Release"
unless this change is applied.

Differential Revision: https://reviews.freebsd.org/D44748

Fixes: d697653cffd19ddaf3cdee2589a830c72ab39777

Approved by: bapt