kbuild: fix build error of 'make nsdeps' in clean tree

Running 'make nsdeps' in a clean source tree fails as follows:

$ make -s clean; make -s defconfig; make nsdeps
   [ snip ]
awk: fatal: cannot open file `init/modules.order' for reading (No such file or directory)
make: *** [Makefile;1307: modules.order] Error 2
make: *** Deleting file 'modules.order'
make: *** Waiting for unfinished jobs....

The cause of the error is 'make nsdeps' does not build modules at all.
Set KBUILD_MODULES to fix it.

Reviewed-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>