Remove unnecessary static specifier that triggers warnings (#737)

src/ngraph/util.hpp

@@ -57,7 +57,7 @@ namespace ngraph
     }
 
     template <typename T>
-    static std::string vector_to_string(const T& v)
+    std::string vector_to_string(const T& v)
     {
         std::ostringstream os;
         os << "[ " << ngraph::join(v) << " ]";