iflib: Make if_shared_ctx_t a pointer to const

[markj]

Summary

This structure is shared among multiple instances of a driver, so we
should ensure that it doesn't somehow get treated as if there's a
separate instance per interface. This is especially important for
software-only drivers like wg.

We could perhaps make DEVICE_REGISTER return a pointer to const as well,
so the sctx structures themselves could be defined as const. I don't
quite understand what this method is supposed to do in the general case
though, so I didn't modify it.

[markj]

sys/net/iflib.h

52

This is the meat of the change, the rest is required to compile after.