kbuild: tar-pkg: enable communication with jobserver

The buildtar script might want to invoke a make, so tell the parent
make to pass the jobserver token pipe to the subcommand by prefixing
the command with a +.

This addresses the issue seen here:

  /bin/sh ../scripts/package/buildtar tar-pkg
  make[3]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

See https://www.gnu.org/software/make/manual/html_node/Job-Slots.html
for more information.
Signed-off-by: Trevor Bourget <tgb.kernel@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/package/Makefile

@@ -104,7 +104,7 @@ clean-dirs += $(objtree)/snap/
 # ---------------------------------------------------------------------------
 tar%pkg: FORCE
 	$(MAKE) -f $(srctree)/Makefile
-	$(CONFIG_SHELL) $(srctree)/scripts/package/buildtar $@
+	+$(CONFIG_SHELL) $(srctree)/scripts/package/buildtar $@
 
 clean-dirs += $(objtree)/tar-install/