UAPI: elf_read_implies_exec() is a kernel-only feature - so hide from userspace

elf_read_implies_exec() is a kernel-only feature as the second parameter is a
constant that isn't exported to userspace.  Not only that, but the
arch-specific overrides are not exported either.

So hide the macro from userspace.

Similarly, struct file should not be predeclared in userspace.

Signed-off-by: David Howells <dhowells@redhat.com>