Fix manpage section of perl module documentation
On Debian (at least) perl documentation is supposed to be installed in
section 3pm. With the build system hardcoding this to 3 instead this
results in a warning by the Debian package linter:
W: public-inbox: wrong-manual-section usr/share/man/man3/PublicInbox::Git.3.gz:74 3 != 3pm
W: public-inbox: wrong-manual-section usr/share/man/man3/PublicInbox::Import.3.gz:74 3 != 3pm
W: public-inbox: wrong-manual-section usr/share/man/man3/PublicInbox::SaPlugin::ListMirror.3.gz:74 3 != 3pm
W: public-inbox: wrong-manual-section ... use --no-tag-display-limit to see all (or pipe to a file/program)
So use "$(MAN3EXT)" instead of "3".
(cherry picked from commit
cd12cf22c4d161b7aec819f45628b983f0bd675d)