sysutils/nut-devel: Build nut-scanner(8)

Depend on devel/ltdl and ensure that ${LOCALBASE}/include and
${LOCALBASE}/lib are available for ./configure to detect ltdl.h and
libltdl to facilitate building nut-scanner(8).

PR: 259153
Reported by: emaste

(cherry picked from commit 2f65a74419f81de566ea103f8e2d8c854d211b24)