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

kernel/sched/sched.h

@@ -1245,6 +1245,7 @@ static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
 
 bool cfs_prio_less(const struct task_struct *a, const struct task_struct *b,
 			bool fi);
+void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi);
 
 /*
  * Helpers to check if the CPU's core cookie matches with the task's cookie