Fix swscanf vswscanf namespace (bug 18542)

[Roland McGrath]

> 	[BZ #18542]
> 	* libio/iovswscanf.c (__vswscanf): Use libc_hidden_def.
> 	(vswscanf) Use ldbl_weak_alias instead of ldbl_strong_alias

Missing a : here.

Otherwise OK.