add nonpoison_mutex feature to root

TODO(connor): Is this necessary?

Author: connortsui20

library/std/src/lib.rs

@@ -407,6 +407,7 @@
 #![feature(edition_panic)]
 #![feature(format_args_nl)]
 #![feature(log_syntax)]
+#![feature(nonpoison_mutex)]
 #![feature(test)]
 #![feature(trace_macros)]
 // tidy-alphabetical-end