criu/build: include msg.mk in files that use msg-*

This patch fixes accidental dropping of msg-gen, msg-link messages.
Just run make with another warn enabled:
[criu]$ make --warn-undefined-variable 2>&1 | grep msg
  PBCC     images/ipc-msg.pb-c.c
  DEP      images/ipc-msg.pb-c.d
  CC       images/ipc-msg.o
Makefile.config:17: warning: undefined variable 'msg-gen'
Makefile.version:20: warning: undefined variable 'msg-gen'
Makefile:120: warning: undefined variable 'msg-link'
Makefile:149: warning: undefined variable 'msg-link'
Makefile:15: warning: undefined variable 'msg-gen'

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>