patch 8.1.1505: running "make clean" twice gives errors

Problem:    Running "make clean" twice gives errors.
Solution:   Add "-f" to "rm". (closes #4516)
diff --git a/src/version.c b/src/version.c
index 82e9e38..13db304 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1505,
+/**/
     1504,
 /**/
     1503,