kbuild: do not run depmod for 'make modules_sign'

Commit 961ab4a ("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: 961ab4a ("kbuild: merge scripts/Makefile.modsign to scripts/Makefile.modinst")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Makefile

@@ -1877,7 +1877,9 @@ quiet_cmd_depmod = DEPMOD  $(MODLIB)
 
 modules_install:
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
+ifndef modules_sign_only
 	$(call cmd,depmod)
+endif
 
 else # CONFIG_MODULES