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/drawline.c b/src/drawline.c
index 6a5c28d..19b0573 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -354,6 +354,8 @@
 #if defined(FEAT_DIFF) || defined(FEAT_SIGNS)
     int		filler_lines = 0;	// nr of filler lines to be drawn
     int		filler_todo = 0;	// nr of filler lines still to do + 1
+#else
+# define filler_lines 0
 #endif
 #ifdef FEAT_DIFF
     hlf_T	diff_hlf = (hlf_T)0;	// type of diff highlighting
@@ -1107,10 +1109,7 @@
 		// Display the absolute or relative line number. After the
 		// first fill with blanks when the 'n' flag isn't in 'cpo'
 		if ((wp->w_p_nu || wp->w_p_rnu)
-			&& (row == startrow
-#ifdef FEAT_DIFF
-			    + filler_lines
-#endif
+			&& (row == startrow + filler_lines
 			    || vim_strchr(p_cpo, CPO_NUMCOL) == NULL))
 		{
 #ifdef FEAT_SIGNS
@@ -1127,11 +1126,7 @@
 #endif
 		    {
 		      // Draw the line number (empty space after wrapping).
-		      if (row == startrow
-#ifdef FEAT_DIFF
-			    + filler_lines
-#endif
-			    )
+		      if (row == startrow + filler_lines)
 		      {
 			long num;
 			char *fmt = "%*ld ";
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,