debian/: Add .gitignore

Ignores these files, which are generated by dpkg-buildpackage -uc -b

 debian/.debhelper/
 debian/build/
 debian/disobedience.debhelper.log
 debian/disobedience.postinst.debhelper
 debian/disobedience.postrm.debhelper
 debian/disobedience.substvars
 debian/disorder-gstreamer.debhelper.log
 debian/disorder-gstreamer.substvars
 debian/disorder-playrtp.debhelper.log
 debian/disorder-playrtp.substvars
 debian/disorder-server.debhelper.log
 debian/disorder-server.postinst.debhelper
 debian/disorder-server.postrm.debhelper
 debian/disorder-server.preinst.debhelper
 debian/disorder-server.prerm.debhelper
 debian/disorder-server.substvars
 debian/disorder.debhelper.log
 debian/disorder.substvars
 debian/tmp/

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>