um: Fix -Wmissing-prototypes warnings for __warp_* and foo

um: Fix -Wmissing-prototypes warnings for __warp_* and foo

These functions are not called explicitly. Let's just workaround
the -Wmissing-prototypes warnings by declaring them locally similar
to what was done in arch/x86/kernel/asm-offsets_32.c.

This will address below -Wmissing-prototypes warnings:

./arch/x86/um/shared/sysdep/kernel-offsets.h:9:6: warning: no previous prototype for ‘foo’ [-Wmissing-prototypes]
arch/um/os-Linux/main.c:187:7: warning: no previous prototype for ‘__wrap_malloc’ [-Wmissing-prototypes]
arch/um/os-Linux/main.c:208:7: warning: no previous prototype for ‘__wrap_calloc’ [-Wmissing-prototypes]
arch/um/os-Linux/main.c:222:6: warning: no previous prototype for ‘__wrap_free’ [-Wmissing-prototypes]
arch/x86/um/user-offsets.c:17:6: warning: no previous prototype for ‘foo’ [-Wmissing-prototypes]

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