Avoid opening the Kokkos namespace (#2800)

This is a nitpick.  Qualify the reduction_identity specialization
instead of opening the Kokkos namespace.

With that change, one can grep for "namespace Kokkos {" to find all
naughty code.

Signed-off-by: Damien L-G <dalg24@gmail.com>

Author: dalg24

common/src/KokkosKernels_Utils.hpp

@@ -1292,16 +1292,14 @@ KOKKOS_INLINE_FUNCTION T *alignPtrTo(InPtr *p) {
 }  // namespace KokkosKernels
 
 // Define the identity for array_sum_reduce
-namespace Kokkos {
 template <typename scalar_t, int N>
-struct reduction_identity<KokkosKernels::Impl::array_sum_reduce<scalar_t, N>> {
+struct Kokkos::reduction_identity<KokkosKernels::Impl::array_sum_reduce<scalar_t, N>> {
   typedef KokkosKernels::Impl::array_sum_reduce<scalar_t, N> T;
   KOKKOS_FORCEINLINE_FUNCTION static T sum() {
     // default constructor default-initializes each element (this should always
     // be 0)
     return T();
   }
 };
-}  // namespace Kokkos
 
 #endif