Create nightingale.1.in

The current build process creates a formatted manpage that is automatically gzipped. I had experienced a few layout errors, at least on the top line of the manpage. And providing source-only manpages seems to be the norm nowadays anyway. Furthermore an already gzipped manpage can't be installed through debhelpers manpage installation command. Instead you need to add the path to the manpage source file in debian/manpages or debian/PACKAGE.manpages.
In this case here debian/rules only adds the nightingale version number to the manpage.