In the case of multi-core compilation, the warning prompts to add a "+" sign.
````
warning: jobserver unavailable: using -j1. Add `+' to parent make rule.
`````
Signed-off-by: Zxl hhyccc <zxlhhy@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16598
Signed-off-by: Robert Marko <robimarko@gmail.com>