Use all cores during build

[Kofhein]

Hello, I mentioned that flutter-elinux compiles much longer than flutter. I've checked what's the difference in terms of build and found that elinux uses cmake command , while flutter uses ninja ( which utilizes all cores by default ). This commit adds all cores utilization. You can omit it if it was done intentionally.

[HidenoriMatsubayashi]

Thank you for sending the PR. Can you please update https://github.com/sony/flutter-elinux/blob/main/AUTHORS too?

[Kofhein]

Sure, updated.