Our general source code depends on headers
generated during protobuf library building
but if library is already built and *.ch
files are removed we might hit a problem
where dep files can't be generated.

Thus add explicit rule pointing out that
library depends on generated *.ch files.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>