headers: Beautify sigframe.h

- include "protobuf/core.pb-c.h" for CoreEntry definition
- gather structure predeclaration on top
- move general comment on top as well
- add extern for construct_sigframe helper

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>