um: Fix the -Wmissing-prototypes warning for get_thread_reg

The get_thread_reg function is defined in the user code, and is
called by the kernel code. It should be declared in a shared header.

Fixes: dbba7f70 ("um: stop polluting the namespace with registers.h contents")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>