debian/rules: Use dh_perl -d

By informing Debhelper that the mosh wrapper script only uses modules in
perl-base (dh_perl -d), we cause ${perl:Depends} to change from ‘perl’
to empty, which silences this build warning:

dpkg-gencontrol: warning: package mosh: unused substitution variable ${perl:Depends}

Signed-off-by: Anders Kaseorg <andersk@mit.edu>