MAINTAINERS: add Gary Guo to pin-init

Gary has been involved in pin-init since the very beginning. He created
his own version before I even joined Rust-for-Linux, contributed ideas
to my solution, and reviewed it. With the removal of the declarative
macro mess, he is also going to be a maintainer of pin-init.

Acked-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

MAINTAINERS

@@ -22920,6 +22920,7 @@ F:	rust/kernel/num/
 
 RUST [PIN-INIT]
 M:	Benno Lossin <lossin@kernel.org>
+M:	Gary Guo <gary@garyguo.net>
 L:	rust-for-linux@vger.kernel.org
 S:	Maintained
 W:	https://rust-for-linux.com/pin-init