Kbuild: Fix deb-pkg target to work with kernel versions ending with -<text-without-digit>

If CONIFIG_LOCALVERSION is set for example to -loop, the following error
message was generated.

dpkg-deb - error: Debian revision (`loop') doesn't contain any digits
dpkg-deb: 1 errors in control file

The patch solves this by adding a numeric revision to package version.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>