add function stack_push

just like the tittle. Thanks for the review. ;D

stack.c

@@ -24,7 +24,7 @@ void stack_push(stack *s, int value)
     }
     
     s->top++;
-    s->data[s->top] = val;
+    s->data[s->top] = value;
 }
 
 void stack_pop(stack *s, int *value) {