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

include/linux/perf_event.h

@@ -1544,4 +1544,8 @@ int perf_event_exit_cpu(unsigned int cpu);
 #define perf_event_exit_cpu	NULL
 #endif
 
+extern void __weak arch_perf_update_userpage(struct perf_event *event,
+					     struct perf_event_mmap_page *userpg,
+					     u64 now);
+
 #endif /* _LINUX_PERF_EVENT_H */