um: Fix the -Wmissing-prototypes warning for __switch_mm

The __switch_mm function is defined in the user code, and is called
by the kernel code. It should be declared in a shared header.

Fixes: 4dc706c2 ("um: take um_mmu.h to asm/mmu.h, clean asm/mmu_context.h a bit")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/asm/mmu.h

@@ -14,8 +14,6 @@ typedef struct mm_context {
 	struct uml_arch_mm_context arch;
 } mm_context_t;
 
-extern void __switch_mm(struct mm_id * mm_idp);
-
 /* Avoid tangled inclusion with asm/ldt.h */
 extern long init_new_ldt(struct mm_context *to_mm, struct mm_context *from_mm);
 extern void free_ldt(struct mm_context *mm);

arch/um/include/shared/skas/mm_id.h

@@ -15,4 +15,6 @@ struct mm_id {
 	int kill;
 };
 
+void __switch_mm(struct mm_id *mm_idp);
+
 #endif