patch 8.0.0651: build failure without the auto command feature

Problem:    Build failure without the auto command feature.
Solution:   Add #ifdef. (closes #1782)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index adf1e3b..bb9a927 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -6832,9 +6832,11 @@
     char_u	*rt;
     int		mustfree;
 
+#ifdef FEAT_AUTOCMD
     /* Set filetype to "help" if still needed. */
     if (STRCMP(curbuf->b_p_ft, "help") != 0)
 	set_option_value((char_u *)"ft", 0L, (char_u *)"help", OPT_LOCAL);
+#endif
 
 #ifdef FEAT_SYN_HL
     if (!syntax_present(curwin))