patch 9.0.1330: handling new value of an option has a long "else if" chain

Problem:    Handling new value of an option has a long "else if" chain.
Solution:   Use a function pointer. (Yegappan Lakshmanan, closes #12015)
diff --git a/src/spellfile.c b/src/spellfile.c
index bf51f5f..a9fa70c 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -1771,7 +1771,7 @@
     // When "zg" was used and the file wasn't loaded yet, should redo
     // 'spelllang' to load it now.
     if (added_word && !didit)
-	did_set_spelllang(curwin);
+	parse_spelllang(curwin);
 }