security/nss: try to fix build in unclean environment

ports like net/mpich may install a mpi.ch in ${LOCALBASE}/include
and hide nss' own lib/freebl/mpi/mpi.h, resulting in build
failures ("unknown type name 'mp_int'").
without a dirty environment of my own to test, this may or may
not fix the issue.

Reported by: Chad Jacob Milios