um: Fix the declaration of vfree

The definition of vfree has changed since commit b3bdda02
("vmalloc: add const to void* parameters"). Update the declaration
of vfree in um_malloc.h to match the latest definition.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>