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>

debian/rules

@@ -23,3 +23,7 @@ override_dh_auto_configure:
 		--enable-ufw \
 		--enable-completion \
 		--enable-compile-warnings=error
+
+override_dh_perl:
+	# mosh only uses Perl modules in perl-base.
+	dh_perl -d