Generate missing inclusion of common.h.

FileGenerator::GenerateBuildDescriptors() emits
"::google::protobuf::internal::OnShutdown(&$shutdownfilename$);" which is
declared in common.h.

Although Protobuf is very well tested a compilation error was triggered while
rolling Protobuf r423:426. There might be something wrong in the Chromium roll
(still in progress).

It seems safer anyway not to rely on indirect headers inclusion.