Merge pull request #1492 from stac47/fix_gcc9_allocator

Fix gcc9 build error test/src/unit-allocator.cpp (Issue #1472)

Author: nlohmann

test/src/unit-allocator.cpp

@@ -110,6 +110,11 @@ struct my_allocator : std::allocator<T>
             p->~T();
         }
     }
+
+    template <class U>
+    struct rebind {
+        using other = my_allocator<U>;
+    };
 };
 
 // allows deletion of raw pointer, usually hold by json_value