make: protobuf -- Generate protofiles deps

Some protobuf files do import another files so
we should generate dependencies and rebuild
the program when needed.

For example core.proto imports core-x86.proto
and core-arm.proto, thus if any of them are
changed the final core.pb-c.c should be regenerated.

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