kbuild: rust: clean `*.long-type-*.txt` files

kbuild: rust: clean `*.long-type-*.txt` files

When rustc prints an error containing a long type that doesn't fit
in a line, it will write the whole thing in a .txt file -- see commit
420dd187e157 (".gitignore: ignore rustc long type txt files") for more
details.

These files are purely compiler artifacts and are not created
intentionally by the build system.

Thus add them to the `clean` target to stop them from cluttering up the
source tree.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1236
Signed-off-by: Joel Kamminga <contact@jkam.dev>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260530184944.10459-1-contact@jkam.dev
[ Reworded and linked to the previous related commit. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>