makeman: Use ${make} instead of plain make to compute ${requireds}

This is currently a no-op but can matter for downstreams whose ${make}
includes additional settings.

Reviewed by: imp, emaste
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D41777

(cherry picked from commit 9c80d66ec1b4c5b9ac7aaf5b0fdbb1628d49c181)