MAINTAINERS: add `.rustfmt.toml` to "RUST" entry

`.rustfmt.toml` is not covered by `MAINTAINERS`, so tools like
`scripts/get_maintainer.pl` do not report it properly, e.g.:

$ scripts/get_maintainer.pl -f .rustfmt.toml
linux-kernel@vger.kernel.org (open list)

It should have been there since the beginning, and while it is not a big
deal since the file has not changed at all since it was added in commit
80db40bac8f4 ("rust: add `.rustfmt.toml`") back in 2022, this will be
especially useful to catch unintended unstable features if upstream
`rustfmt` started to allow them in stable toolchains [1][2].

Thus add it.

Link: https://github.com/rust-lang/rustfmt/issues/6829#issuecomment-4084325200 [1]
Link: https://github.com/rust-lang/rustfmt/issues/4884 [2]
Fixes: 80db40bac8f4 ("rust: add `.rustfmt.toml`")
Link: https://patch.msgid.link/20260323024206.129401-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>