um: Remove the redundant declaration of high_physmem

high_physmem has already been declared in as-layout.h, so there is
no need to declare it explicitly in the .c file again.

While at it, group the declarations of __real_malloc and __real_free
together to make the code slightly more readable.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20240916045950.508910-2-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>