MAINTAINERS: add Boqun, Gary and Björn as reviewers

Link: https://lore.kernel.org/lkml/YuiLM6ca8Ih0WiS3@boqun-archlinux/
Link: https://lore.kernel.org/lkml/20220802155314.0000380e@garyguo.net/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

MAINTAINERS

@@ -17481,6 +17481,9 @@ RUST
 M:	Miguel Ojeda <ojeda@kernel.org>
 M:	Alex Gaynor <alex.gaynor@gmail.com>
 M:	Wedson Almeida Filho <wedsonaf@google.com>
+R:	Boqun Feng <boqun.feng@gmail.com>
+R:	Gary Guo <gary@garyguo.net>
+R:	Björn Roy Baron <bjorn3_gh@protonmail.com>
 L:	rust-for-linux@vger.kernel.org
 S:	Supported
 W:	https://github.com/Rust-for-Linux/linux