kbuild: obj-dirs is calculated incorrectly if hostprogs-y is defined

When Makefile.host is included, $(obj-dirs) is subjected to the
addprefix operation for the second time.  Prefix only needs to be added
to the newly added directories, but not to those that came from
Makefile.lib.

This causes the build system to create unneeded empty directories in the
build tree when building in a separate directory.  For instance,
lib/lib/zlib_inflate is created in the build tree.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>