patch 8.2.4521: build failure without the +diff feature

Problem:    Build failure without the +diff feature. (John Marriott)
Solution:   Define filler+lines if not declaring it.
diff --git a/src/version.c b/src/version.c
index 9753167..c53f4d8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4521,
+/**/
     4520,
 /**/
     4519,