Strip +dev suffix in plain upstream version

Jenkins appends a `+dev<num>.<hash>` suffix to the upstream version
since we apply patches on the main branch instead of via
`debian/patches`. Strip that suffix like Debian does for their `+dfsg`
versioning to get the real upstream version.

https://phabricator.endlessm.com/T34488

debian/rules

@@ -23,7 +23,7 @@ endif
 
 deb_version := $(shell dpkg-parsechangelog | sed -ne "s/^Version: \(.*\)/\1/p")
 upstream_version := $(shell echo $(deb_version) | sed -e "s/-[^-]*$$//")
-plain_upstream_version := $(shell echo $(upstream_version) | sed -e "s/+dfsg.*//")
+plain_upstream_version := $(shell echo $(upstream_version) | sed -e "s/+\(dfsg\|dev\).*//")
 
 DBX_LIST = dbx.esl
 DBX_HASHES = debian/$(distributor)-dbx.hashes