physx: fix compiler errors

panda/src/physx/physxEnums.cxx

@@ -16,11 +16,6 @@
 #include "string_utils.h"
 #include "config_putil.h"
 
-using std::istream;
-using std::ostream;
-
-ostream &
-operator << (ostream &out, PhysxEnums::PhysxUpAxis axis) {
 std::ostream &
 operator << (std::ostream &out, PhysxEnums::PhysxUpAxis axis) {
 
@@ -38,8 +33,8 @@ operator << (std::ostream &out, PhysxEnums::PhysxUpAxis axis) {
   return out << "**invalid PhysxEnums::PhysxUpAxis value: (" << (int)axis << ")**";
 }
 
-istream &
-operator >> (istream &in, PhysxEnums::PhysxUpAxis &axis) {
+std::istream &
+operator >> (std::istream &in, PhysxEnums::PhysxUpAxis &axis) {
 
   std::string word;
   in >> word;