patch 8.0.1175: build failure without +termresponse

Problem:    Build failure without +termresponse.
Solution:   Add #ifdef.

src/syntax.c

@@ -7350,10 +7350,12 @@ lookup_color(int idx, int foreground, int *boldp)
 	    else
 		color = color_numbers_8[idx];
 	}
+#ifdef FEAT_TERMRESPONSE
 	if (t_colors >= 256 && color == 15 && is_mac_terminal)
 	    /* Terminal.app has a bug: 15 is light grey. Use white
 	     * from the color cube instead. */
 	    color = 231;
+#endif
     }
     return color;
 }