Swap out unboxed_closures feature gate for min_specialization

For some reason unboxed_closures supresses the feature gate for
min_specialization when implementing TrustedStep. min_specialization is
the true feature that is used.

Author: bjorn3

compiler/rustc_index/src/lib.rs

@@ -2,7 +2,7 @@
 #![feature(bench_black_box)]
 #![feature(extend_one)]
 #![feature(iter_zip)]
-#![feature(unboxed_closures)]
+#![feature(min_specialization)]
 #![feature(test)]
 #![feature(fn_traits)]