libstdc++: Simplify preprocessor/namespace nesting in <bits/move.h>

There's no good reason to conditionally close and reopen namespace std
within an #if block. Just include the <type_traits> header at the top
instead.

libstdc++-v3/ChangeLog:

	* include/bits/move.h: Simplify opening/closing namespace std.

(cherry picked from commit 5c8b154c)