patch 9.0.1307: setting 'formatoptions' with :let doesn't check for errors

Problem:    Setting 'formatoptions' with :let doesn't check for errors.
Solution:   Pass "errbuf" to set_string_option(). (Yegappan Lakshmanan,
            closes #11974, closes #11972)
diff --git a/src/version.c b/src/version.c
index f3877e4..ee9bf28 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1307,
+/**/
     1306,
 /**/
     1305,