um: Fix the return type of __switch_to

Make it match the declaration in asm-generic/switch_to.h. And
also include the header to allow the compiler to check it.
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/process.c

@@ -26,6 +26,7 @@
 #include <linux/resume_user_mode.h>
 #include <asm/current.h>
 #include <asm/mmu_context.h>
+#include <asm/switch_to.h>
 #include <linux/uaccess.h>
 #include <as-layout.h>
 #include <kern_util.h>
@@ -73,7 +74,7 @@ static inline void set_current(struct task_struct *task)
 
 extern void arch_switch_to(struct task_struct *to);
 
-void *__switch_to(struct task_struct *from, struct task_struct *to)
+struct task_struct *__switch_to(struct task_struct *from, struct task_struct *to)
 {
 	to->thread.prev_sched = from;
 	set_current(to);