libCom: Explicitly use namespace std for size_t declaration in epicsSingleton.h

This fixes a compile error that occurs in new gcc versions (since ~4.6.1)

src/libCom/cxxTemplates/epicsSingleton.h

@@ -34,7 +34,7 @@ public:
     void * pInstance () const;
 private:
     void * _pInstance;
-    size_t _refCount;
+    std :: size_t _refCount;
     SingletonUntyped ( const SingletonUntyped & );
     SingletonUntyped & operator = ( const SingletonUntyped & );
 };