chore: untrack target/ build artifacts

The whole `target/` tree (510 files, including a stale Linux
`libopenvx_ffi.so` that pre-dated the rust workspace move) was
committed before `.gitignore` listed it. `.gitignore` already has
`target/`, so untracking is a one-shot cleanup — `cargo build` will
recreate the directory locally and the gitignore rule will keep it
out from now on. No source / CI changes; CI builds rustVX from
scratch and uploads its own `target/release/libopenvx_ffi.so` as a
workflow artifact, so removing the stale checked-in copy is safe.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: kiritigowda