MAINTAINERS: Add myself as a reviewer for SDL audio

I've got some experience with the SDL library, so I can help
reviewing patches here.

Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211030062106.46024-1-huth@tuxfamily.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

MAINTAINERS

@@ -2315,6 +2315,7 @@ F: audio/paaudio.c
 
 SDL Audio backend
 M: Gerd Hoffmann <kraxel@redhat.com>
+R: Thomas Huth <huth@tuxfamily.org>
 S: Odd Fixes
 F: audio/sdlaudio.c