Make get_global_resource host/device

The function is called from a host/device constructors and may be invoked from the device code.

As it happens neither NVCC nor clang diagnose such an invalid call.
llvm/llvm-project#118415

As the result, in unoptimized builds we end up with ptxas failing with
an unresolved reference to this function because it is never generated
during GPU-side compilation.
#2813 (comment)

thrust/thrust/mr/memory_resource.h

@@ -204,7 +204,7 @@ _CCCL_HOST_DEVICE bool operator!=(const memory_resource<Pointer>& lhs, const mem
  *  \return a pointer to a global instance of \p MR.
  */
 template <typename MR>
-_CCCL_HOST MR* get_global_resource()
+_CCCL_HOST_DEVICE MR* get_global_resource()
 {
   static MR resource;
   return &resource;