Fix zero size debug array swap noexcept qualification

[Jonathan Wakely]

On 23/06/16 22:22 +0200, François Dumont wrote:
>    Debug mode array had simply been forgotten when fixing zero-size 
>swap method as part of swappable traits implementation.
>
>    * include/debug/array (array<>::swap): Fix noexcept qualificaton for
>    zero-size array.
>
>    Tested under Linux x86_64 debug mode.

OK for trunk, thanks.