os-release: Add DISTRO_CODENAME as vardeps for do_compile

DISTRO_CODENAME is part of VERSION variable but not used as dependency
for do_compile task. Append it to the vardeps list to rebuild in case it
changes.

Signed-off-by: Daniel Gomez <daniel@qtec.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>