[PATCH] kbuild: When checking depmod version, redirect stderr

When running depmod to check for the correct version number, extra
output we don't need to see, such as "depmod: QM_MODULES: Function not
implemented" may show up.  Redirect stderr to /dev/null as the version
information that we do care about comes to stdout.

Signed-off-by: Tom Rini <trini@kernel.crashing.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>