lib/mu-msg-file: Use canonical path instead of real path

For the new symlink-support, it's better to use the *canonical* path than
the *realpath(3)* for files, so removing a symlinked maildir will work as
expected.

lib/mu-msg-priv.h

@@ -40,8 +40,8 @@ struct _MuMsgFile {
 	GMimeMessage	*_mime_msg;
 	time_t		 _timestamp;
 	size_t		 _size;
-	char		 _path    [PATH_MAX + 1];
-	char		 _maildir [PATH_MAX + 1];
+	char	         *_path;
+	char		 *_maildir;
 };