sched: Make task_vruntime_update() prototype visible

Having the prototype next to the caller but not visible to the callee causes
a W=1 warning:

kernel/sched/fair.c:11985:6: error: no previous prototype for 'task_vruntime_update' [-Werror=missing-prototypes]

Move this to a header, as we do for all other function declarations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230522195021.3456768-5-arnd@kernel.org