cleanup: Fix missing `std::` prefixes

direct/src/dcparser/dcClass_ext.cxx

@@ -108,7 +108,7 @@ receive_update(PyObject *distobj, DatagramIterator &di) const {
   int field_id = packer.raw_unpack_uint16();
   DCField *field = _this->get_field_by_index(field_id);
   if (field == nullptr) {
-    ostringstream strm;
+    std::ostringstream strm;
     strm
         << "Received update for field " << field_id << ", not in class "
         << _this->get_name();

dtool/src/dtoolutil/iostream_ext.h

@@ -22,6 +22,9 @@
 #include <iostream>
 #include "py_panda.h"
 
+using std::ostream;
+using std::istream;
+
 /**
  * These classes define the extension methods for istream and ostream, which
  * are called instead of any C++ methods with the same prototype.