AK: Use unchecked_append in AK::Stack, as we always validate the size

This removes one additional usage of Vector::append that stops us from
disabling it when compiling the Kernel.

AK/Stack.h

@@ -21,7 +21,7 @@ public:
         if (m_stack.size() >= stack_size)
             return false;
 
-        m_stack.append(item);
+        m_stack.unchecked_append(item);
         return true;
     }
 
@@ -30,7 +30,7 @@ public:
         if (m_stack.size() >= stack_size)
             return false;
 
-        m_stack.append(move(item));
+        m_stack.unchecked_append(move(item));
         return true;
     }