patch 8.1.1215: "make clean" does not remove generated src/po files

Problem:    "make clean" does not remove generated src/po files.
Solution:   Remove the files for "make clean". (Christian Brabandt)
diff --git a/src/version.c b/src/version.c
index e6dc264..0d2e344 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1215,
+/**/
     1214,
 /**/
     1213,