MAINTAINERS: Add myself as futex reviewer

I'm volunteering to help review some of the pain.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210122171101.15991-1-dave@stgolabs.net

MAINTAINERS

@@ -7363,6 +7363,7 @@ M:	Thomas Gleixner <tglx@linutronix.de>
 M:	Ingo Molnar <mingo@redhat.com>
 R:	Peter Zijlstra <peterz@infradead.org>
 R:	Darren Hart <dvhart@infradead.org>
+R:	Davidlohr Bueso <dave@stgolabs.net>
 L:	linux-kernel@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/core