Fix build on latest git master

Jethro Beekman · Jethro Beekman · commit 505167d7c58f · 2018-12-25T13:10:16.000+05:30
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
@@ -314,7 +314,8 @@
 #![feature(alloc_layout_extra)]
 #![feature(maybe_uninit)]
 #![cfg_attr(target_env = "sgx", feature(global_asm, range_contains, slice_index_methods,
-                                        decl_macro, coerce_unsized, sgx_platform))]
+                                        decl_macro, coerce_unsized, sgx_platform,
+                                        min_const_unsafe_fn))]
 
 #![default_lib_allocator]
 
