kbuild: In setlocalversion change -git_dirty to just -dirty

When building Debian packages directly from the git tree, the appended
"git_dirty" is a problem due to the underscore.  In order to cause the
least problems, change that just to "dirty".
Signed-off-by: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/setlocalversion

@@ -17,6 +17,6 @@ if head=`git rev-parse --verify HEAD 2>/dev/null`; then
 
 	# Are there uncommitted changes?
 	if git diff-files | read dummy; then
-		printf '%s' -git_dirty
+		printf '%s' -dirty
 	fi
 fi