kernel/events: Add a missing prototype for arch_perf_update_userpage()

... in order to fix a -Wmissing-prototype warning.

No functional changes.

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200109131351.9468-1-b.thiel@posteo.de