make: Make protobuf target to depend on *.ch

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>

protobuf/Makefile

@@ -61,7 +61,7 @@ $(PROTO_LIBRARY): $(PROTO_OBJS)
 
 .PHONY: protobuf
 
-protobuf: $(PROTO_LIBRARY)
+protobuf: $(PROTO_LIBRARY) $(PROTO_SRCS) $(PROTO_HDRS)
 
 clean:
 	$(E) "  CLEAN PROTOBUF"