kbuild: rust: clean libpin_init_internal in mrproper

kbuild: rust: clean libpin_init_internal in mrproper

When I enabled Rust compilation, I wanted to clean up its output, so I
used make mrproper. However, I was still able to find that
libpin_init_internal.so in the rust directory was not deleted, while
all other corresponding outputs were cleared.

Thus add it to the `MRPROPER_FILES` list.

Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Chen Miao <chenmiao@openatom.club>
Fixes: d7659acca7a3 ("rust: add pin-init crate build infrastructure")
Cc: stable@vger.kernel.org
Acked-by: Nicolas Schier <nsc@kernel.org>
Acked-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/71ff222b8731e63e06059c5d8566434e508baf2b.1761876365.git.chenmiao@openatom.club
[ Fixed tags and Git author as discussed. Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>