build: Add missing USDT header dependency to kernel

[theuni]

Noticed while testing a branch that replaces boost::multi_index with a custom replacement.

Currently depends builds pick up usdt and boost from the same path, and because boost always exists, the usdt path is implicitly included. So without boost, USDT isn't found.

An alternative to this would be to disable USDT for the kernel. I'd be open to either approach.