add missing debian contrib files to tarball

Makefile.am

@@ -41,6 +41,7 @@ DIST_DOCS = $(wildcard doc/*.md) $(wildcard doc/release-notes/*.md)
 DIST_CONTRIB = $(top_srcdir)/contrib/bitcoin-cli.bash-completion \
 	       $(top_srcdir)/contrib/bitcoin-tx.bash-completion \
 	       $(top_srcdir)/contrib/bitcoind.bash-completion \
+	       $(top_srcdir)/contrib/debian/copyright \
 	       $(top_srcdir)/contrib/init \
 	       $(top_srcdir)/contrib/install_db4.sh \
 	       $(top_srcdir)/contrib/rpm