MAINTAINERS: Add myself as context tracking maintainer

I've been missing a lot of patches touching context tracking for which
I wasn't Cc'ed these last months. The code looks like a simple single
file but has a lot of subtle tentacles.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210512232924.150322-11-frederic@kernel.org