maintainers: Add myself as linux-user reviewer

I volunteer to review linux-user patches.
Adding myself will help to not miss some of them.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Acked-by: Riku Voipio <riku.voipio@linaro.org>
Message-id: 20170510153950.29343-1-laurent@vivier.eu
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

MAINTAINERS

@@ -1578,6 +1578,7 @@ F: default-configs/*-bsd-user.mak
 
 Linux user
 M: Riku Voipio <riku.voipio@iki.fi>
+R: Laurent Vivier <laurent@vivier.eu>
 S: Maintained
 F: linux-user/
 F: default-configs/*-linux-user.mak