glcpp: Fix a case of == where = probably ought to be.

Caught by a GCC warning.
diff --git a/glcpp/glcpp-parse.y b/glcpp/glcpp-parse.y
index 807cf59..ede2bb8 100644
--- a/glcpp/glcpp-parse.y
+++ b/glcpp/glcpp-parse.y
@@ -1356,7 +1356,7 @@
 				else
 					list->head = last->next;
 				if (last == list->tail)
-					list->tail == NULL;
+					list->tail = NULL;
 			}
 		} else {
 			node_prev = node;