patch 8.1.0510: filter test fails when $LANG is C.UTF-8

Problem:    Filter test fails when $LANG is C.UTF-8.
Solution:   Set 'helplang' to "en" for any C language. (Christian Brabandt,
            closes #3577)
diff --git a/src/version.c b/src/version.c
index fb8a16d..a5b6d87 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    510,
+/**/
     509,
 /**/
     508,