perf/core: Make perf_swevent_init_cpu() static

'make W=1' causes GCC to complain:

  kernel/events/core.c:11877:6: warning: no previous prototype for 'perf_swevent_init_cpu' [-Wmissing-prototypes]

It's not referenced anywhere else, make it static.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/28974.1552377997@turing-police


Signed-off-by: Ingo Molnar <mingo@kernel.org>