patch 8.2.0851: can't distinguish <M-a> from accented "a" in the GUI

Problem:    Can't distinguish <M-a> from accented "a" in the GUI.
Solution:   Use another way to make mapping <C-bslash> work. (closes #6163)
diff --git a/src/version.c b/src/version.c
index 6fb570d..da31699 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    851,
+/**/
     850,
 /**/
     849,