MAINTAINERS: put myself forward for gdbstub

As I've been reviewing a lot of this recently and I'm going to put
together a pull request I'd better keep an eye on it. Philippe has
also volunteered to be a reviewer.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 588c8d9..acbad13 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1865,7 +1865,9 @@
 F: util/qemu-error.c
 
 GDB stub
-S: Orphan
+M: Alex Bennée <alex.bennee@linaro.org>
+R: Philippe Mathieu-Daudé <philmd@redhat.com>
+S: Maintained
 F: gdbstub*
 F: gdb-xml/