kbuild: fix -I option expansion with O=... builds

When adding extra -I options with O=... we could
end up in a situation where there were no parameters to -I.
So we had a commandline that looked like this:

    ... -I -Wall ...

This had the undesired side effect that gcc assumed "-Wall"
was a path to look for include files so this options was
effectively ignored.

This happens only when we build the generated module.mod.c files
as part of the final modules builds and is as such harmless
with current kbuild.
This bug was exposed when we rearranged the options to gcc.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>