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>

arch/um/include/shared/um_malloc.h

@@ -12,7 +12,7 @@ extern void *uml_kmalloc(int size, int flags);
 extern void kfree(const void *ptr);
 
 extern void *vmalloc(unsigned long size);
-extern void vfree(void *ptr);
+extern void vfree(const void *ptr);
 
 #endif /* __UM_MALLOC_H__ */