make: Fix build bug introduced in 726a1180

In 726a1180 we made protobuf library to depend
on *.ch which is good thing but a bit incomplete.
We need a rule to generate headers if they are
missed.

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