MAINTAINERS: add Rust FW abstractions to FIRMWARE LOADER

Add missing file path of the Rust abstractions to the maintainers entry,
until we can move it to 'drivers/base/firmware_loader/'.

CC: Luis Chamberlain <mcgrof@kernel.org>
CC: Russ Weight <russ.weight@linux.dev>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://lore.kernel.org/r/20240619132029.59296-2-dakr@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -8573,6 +8573,7 @@ L:	linux-kernel@vger.kernel.org
 S:	Maintained
 F:	Documentation/firmware_class/
 F:	drivers/base/firmware_loader/
+F:	rust/kernel/firmware.rs
 F:	include/linux/firmware.h
 
 FLEXTIMER FTM-QUADDEC DRIVER