kbuild: modpost: remove unnecessary dependency for __modpost

__modpost is a phony target. The dependency on FORCE is pointless.
All the objects have been built in the previous stage, so the
dependency on the objects are not necessary either.

Count the number of modules in a more straightforward way.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>