patch 8.1.0311: filtering entries in a quickfix list is not easy

Problem:    Filtering entries in a quickfix list is not easy.
Solution:   Add the cfilter plugin. (Yegappan Lakshmanan)
diff --git a/src/version.c b/src/version.c
index cc62660..f37cdfc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    311,
+/**/
     310,
 /**/
     309,