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.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4521,
 /**/
     4520,
 /**/