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>