common/: add support for changelog field.

This commits allows ./xbps-src show to show the changelog field and for
changelog to be included with the -c/--changelog option via xbps-create.

Closes: #14102 [via git-merge-pr]

common/xbps-src/shutils/show.sh

@@ -15,6 +15,7 @@ show_pkg() {
     [ -n "$noarch" ] && echo "noarch:		yes"
     echo "maintainer:	$maintainer"
     [ -n "$homepage" ] && echo "Upstream URL:	$homepage"
+    [ -n "$changelog" ] && echo "Changelog URL:	$changelog"
     [ -n "$license" ] && echo "License(s):	$license"
     [ -n "$build_style" ] && echo "build_style:	$build_style"
     for i in ${configure_args}; do