Add const_ptr_null feature

ids1024 · ids1024 · commit 1df1ef789ceb · 2017-09-17T18:56:57.000-07:00
This seems to be required now, due to rust-lang/rust#43017
diff --git a/newlib/libc/sys/redox/src/lib.rs b/newlib/libc/sys/redox/src/lib.rs
@@ -5,6 +5,7 @@
     alloc_system,
     compiler_builtins_lib,
     const_fn,
+    const_ptr_null,
     core_intrinsics,
     drop_types_in_const,
     global_allocator,
