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>