Fix default implementation of has_push_back and has_push_front

[SVN r54948]

test/Jamfile.v2

@@ -66,6 +66,7 @@ compile numeric_ops.cpp ;
 compile pair_view.cpp ;
 compile partition.cpp ;
 compile pop_front.cpp ;
+compile push_back.cpp ;
 compile push_front.cpp ;
 compile quote.cpp ;
 compile range_c.cpp ;