caps.override: fix compiler warning

Re-use the already-extracted caps and structure pointers, instead
of getting them again, thus fixing:
gstcaps.override: In function 'pygst_caps_sq_item':
gstcaps.override:361:16: error: variable 'structure' set but not used

gst/gstcaps.override

@@ -370,7 +370,7 @@ pygst_caps_sq_item(PyObject *self, Py_ssize_t i)
 				    
 	/* pyg_boxed_new handles NULL checking */
 	ret = pyg_boxed_new(GST_TYPE_STRUCTURE, 
-	                    gst_caps_get_structure(pyg_boxed_get(self, GstCaps), i), 
+	                    structure,
 			    FALSE, FALSE);
 	if (ret)
 	  pygst_caps_map_add (ret, self);