gccrs: ffi-polonius: Remove usage of extern types.

gccrs: ffi-polonius: Remove usage of extern types.

This will allow us to revert our dependency on extern types, which would
help our godbolt build as well as our various builders.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/ffi-polonius/src/gccrs_ffi.rs: Remove extern
	type feature.
	* checks/errors/borrowck/ffi-polonius/src/lib.rs: Define FFIVector
	per the nomicon's recommendation
	https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs