MAINTAINERS: add io_uring tool to IO_URING

The files in ./tools/io_uring/ are maintained by the IO_URING maintainers.
Reflect that fact in MAINTAINERS.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210505053728.3868-1-lukas.bulwahn@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -9497,6 +9497,7 @@ F:	fs/io-wq.h
 F:	fs/io_uring.c
 F:	include/linux/io_uring.h
 F:	include/uapi/linux/io_uring.h
+F:	tools/io_uring/
 
 IPMI SUBSYSTEM
 M:	Corey Minyard <minyard@acm.org>