Make checkdeps.py check third_party.

third_party/protobuf/src/google/protobuf/compiler/plugin.pb.h

 // Generated by the protocol buffer compiler.  DO NOT EDIT!
 // source: google/protobuf/compiler/plugin.proto
 
 #ifndef PROTOBUF_google_2fprotobuf_2fcompiler_2fplugin_2eproto__INCLUDED
 #define PROTOBUF_google_2fprotobuf_2fcompiler_2fplugin_2eproto__INCLUDED
 
 #include <string>
 
 #include <google/protobuf/stubs/common.h>
 
 #if GOOGLE_PROTOBUF_VERSION < 2005000
 #error This file was generated by a newer version of protoc which is
 #error incompatible with your Protocol Buffer headers.  Please update
 #error your headers.
 #endif
 #if 2005000 < GOOGLE_PROTOBUF_MIN_PROTOC_VERSION
 #error This file was generated by an older version of protoc which is
 #error incompatible with your Protocol Buffer headers.  Please
 #error regenerate this file with a newer version of protoc.
 #endif
 
 #include <google/protobuf/generated_message_util.h>
 #include <google/protobuf/message.h>
 #include <google/protobuf/repeated_field.h>
 #include <google/protobuf/extension_set.h>
 #include <google/protobuf/unknown_field_set.h>
-#include "google/protobuf/descriptor.pb.h"
+#include <google/protobuf/descriptor.pb.h>
 // @@protoc_insertion_point(includes)
 
 namespace google {
 namespace protobuf {
 namespace compiler {
 
 // Internal implementation detail -- do not call these.
 void LIBPROTOC_EXPORT protobuf_AddDesc_google_2fprotobuf_2fcompiler_2fplugin_2eproto();
 void protobuf_AssignDesc_google_2fprotobuf_2fcompiler_2fplugin_2eproto();
 void protobuf_ShutdownFile_google_2fprotobuf_2fcompiler_2fplugin_2eproto();
@@ skipping 809 matching lines @@
 namespace protobuf {
 
 
 }  // namespace google
 }  // namespace protobuf
 #endif  // SWIG
 
 // @@protoc_insertion_point(global_scope)
 
 #endif  // PROTOBUF_google_2fprotobuf_2fcompiler_2fplugin_2eproto__INCLUDED