kbuild: add a missing line for help message

The help message line for building the source RPM package was missing.
Added it.

Signed-off-by: Jun ASAKA <JunASAKA@zzy040330.moe>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
1 file changed