kbuild: Fix comment in Makefile.lib

KBUILD_MODNAME is not defined for files that are linked into multiple
modules, and trying to change reality to match documentation would
result in all sorts of trouble. E.g. options for built-in modules would
be called either foo_bar.param, foo.param, or bar.param, depending on
the configuration. So just change the comment.

Signed-off-by: Michal Marek <mmarek@suse.cz>