MAINTAINERS: Add Frederic Weisbecker as nohz/dyntics maintainer

Frederic has been improving and maintaining the nohz/dynticks kernel features
for years, so make his de facto maintainership official.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>