kbuild: do not run depmod for 'make modules_sign'

Commit 961ab4a3 ("kbuild: merge scripts/Makefile.modsign to
scripts/Makefile.modinst") started to run depmod at the end of
'make modules_sign'.

Move the depmod rule to scripts/Makefile.modinst and run it only when
$(modules_sign_only) is empty.

Fixes: 961ab4a3

 ("kbuild: merge scripts/Makefile.modsign to scripts/Makefile.modinst")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>