update git-version-gen to generate proper version numbers

We don't want the version of the last tagged version, but the version
number uniquely representing the current HEAD.  Use the script from
libosmocore.

I suspect that this somehow got broken in commit 00d5114717

Related: OS#3517
Change-Id: Iba3212aa417dce4240c5c27eb4f12afcd9c95e5b

git-version-gen

@@ -93,7 +93,8 @@ if test -n "$v"
 then
     : # use $v
 elif
-       v=`git tag -l --sort=v:refname | grep "^[0-9]*.[0-9]*.[0-9]*$" | tail -n 1 2>/dev/null` \
+       v=`git describe --abbrev=4 --match='v*' HEAD 2>/dev/null \
+	  || git describe --abbrev=4 HEAD 2>/dev/null` \
     && case $v in
 	 [0-9]*) ;;
 	 v[0-9]*) ;;