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

No functional changes.
Signed-off-by: NBenjamin Thiel <b.thiel@posteo.de>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200109131351.9468-1-b.thiel@posteo.de