pinned: Add noexcept

The reference operations can't have any exceptions.

Change-Id: If4e37b2497196b26d5732dc29325b94240927bca
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/stdplus/pinned.hpp b/include/stdplus/pinned.hpp
index 851ead8..b3df00d 100644
--- a/include/stdplus/pinned.hpp
+++ b/include/stdplus/pinned.hpp
@@ -20,11 +20,11 @@
     using type = struct
     {
         T v;
-        constexpr operator T&()
+        constexpr operator T&() noexcept
         {
             return v;
         }
-        constexpr operator const T&() const
+        constexpr operator const T&() const noexcept
         {
             return v;
         }