(mh_file_to_message): Change signature

@@ -239,7 +239,7 @@ void mh_init (void);
 void mh_init2 (void);
 void mh_read_profile (void);
 int mh_read_formfile (char *name, char **pformat);
-mu_message_t mh_file_to_message (char *folder, char *file_name);
+mu_message_t mh_file_to_message (const char *folder, char *file_name);
 mu_message_t mh_stream_to_message (mu_stream_t stream);
 void mh_install (char *name, int automode);
 

@@ -566,7 +566,7 @@ mh_file_copy (const char *from, const char *to)
 }
 
 mu_message_t
-mh_file_to_message (char *folder, char *file_name)
+mh_file_to_message (const char *folder, char *file_name)
 {
   struct stat st;
   int rc;