pinned: Declare within namespace

We must can only define with the inline format, to declare we need to do
this within a proper namespace block.

Change-Id: I861020e78554c48b133098c06347fb3ba706ec7d
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/stdplus/pinned.hpp b/include/stdplus/pinned.hpp
index e52a57f..851ead8 100644
--- a/include/stdplus/pinned.hpp
+++ b/include/stdplus/pinned.hpp
@@ -102,6 +102,8 @@
 
 } // namespace stdplus
 
+namespace std
+{
 template <class T>
-typename stdplus::Pinned<T>&&
-    std::move(stdplus::Pinned<T>& t) noexcept = delete;
+typename stdplus::Pinned<T>&& move(stdplus::Pinned<T>& t) noexcept = delete;
+}