patch 8.0.0431: 'cinoptions' cannot set indent for extern block

Problem:    'cinoptions' cannot set indent for extern block.
Solution:   Add the "E" flag in 'cinoptions'. (Hirohito Higashi)
diff --git a/src/structs.h b/src/structs.h
index 5745df2..475280a 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2236,6 +2236,7 @@
     int		b_ind_hash_comment;
     int		b_ind_cpp_namespace;
     int		b_ind_if_for_while;
+    int		b_ind_cpp_extern_c;
 #endif
 
     linenr_T	b_no_eol_lnum;	/* non-zero lnum when last line of next binary