THRIFT-3317: C++ - generated code will properly reference global apache namespace

Client: C++
Patch: Carrey Zhan

apache::xxxxx --> ::apache::xxxxx
diff --git a/compiler/cpp/src/generate/t_cpp_generator.cc b/compiler/cpp/src/generate/t_cpp_generator.cc
index 6577b82..4e03d94 100644
--- a/compiler/cpp/src/generate/t_cpp_generator.cc
+++ b/compiler/cpp/src/generate/t_cpp_generator.cc
@@ -1539,7 +1539,7 @@
 
   indent_up();
 
-  out << indent() << "using apache::thrift::to_string;" << endl;
+  out << indent() << "using ::apache::thrift::to_string;" << endl;
   out << indent() << "out << \"" << tstruct->get_name() << "(\";" << endl;
   struct_ostream_operator_generator::generate_fields(out, tstruct->get_members(), indent());
   out << indent() << "out << \")\";" << endl;