patch 8.0.1215: newer gcc warns for implicit fallthrough

Problem:    Newer gcc warns for implicit fallthrough.
Solution:   Consistently use a FALLTHROUGH comment. (Christian Brabandt)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index ea6df86..036569f 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4115,7 +4115,7 @@
 	case CMD_bunload:
 	    while ((xp->xp_pattern = vim_strchr(arg, ' ')) != NULL)
 		arg = xp->xp_pattern + 1;
-	    /*FALLTHROUGH*/
+	    /* FALLTHROUGH */
 	case CMD_buffer:
 	case CMD_sbuffer:
 	case CMD_checktime: