perf: Update MAINTAINERS for x86 move

The move of the x86 perf implementation forgot to update the MAINTAINERS
F(ile) pattern.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: fa9cbf32

 ("perf/x86: Move perf_event.c ............... => x86/events/core.c")
Link: http://lkml.kernel.org/r/20160519103019.GJ3206@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>