Add a dummy build recipe for the proto generated header files

This fixed issue that gnumake skip updating the cpp file that includes
the generated header file when the .proto file gets updated.
For example:
Say a.cc includes b.pb.h, since b.pb.h is just byproduct of the rule
that generates b.pb.cc, and though we have dependency "b.pb.h :
b.pb.cc", but we don't have build recipe for that rule.
Gmake stupidly thinks that b.pb.h must not be updated in that case so
it skips all targets that depends on b.pb.h!
With the dumy build recipe, gmake now doesn't skip the depedent targets.

Bug: 13009798
Change-Id: I39adc09b7656bdd023f578fb8933667944fd974c