Added Linux ARPHRD_CAN constant

[fpagliughi]

The Linux ARPHRD_CAN constant (280) for CANbus appears to be missing.

[rust-highfive]

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[JohnTitor]

Seems some targets' (e.g. Android, Linux) tests need to import linux/if_arp.h.

[fpagliughi]

OK. I'll have a look.

[JohnTitor]

@fpagliughi Do you need help? If you're busy I'm happy to work on it instead :)

[JohnTitor]

@fpagliughi Because we need some test tweaks, I opened #2215 with additional changes, hope you're fine with it. Thanks for the PR anyway!

[fpagliughi]

Ha. Yeah, awesome. I wasn't worried about my one line PR, just apologies I didn't follow up. I got bogged down in a couple other projects. Glad it got in there.