patch 8.0.0655: not easy to make sure a function does not exist

Problem:    Not easy to make sure a function does not exist.
Solution:   Add ! as an optional argument to :delfunc.
diff --git a/src/version.c b/src/version.c
index 1e7d648..533660f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    655,
+/**/
     654,
 /**/
     653,