Bound Any with 'static

This bound is already implicit through the AnyPrivate trait,
but since it is not explicit, you still have to write Box<Any + 'static>,
even though Any can only be 'static.

Introducing the 'static bound here makes this bound explicit, making
Box<Any> legal.
auto merge of #17132 : reem/rust/any-static-bound, r=alexcrichton

This bound is already implicit through the AnyPrivate trait,
but since it is not explicit, you still have to write Box<Any + 'static>,
even though Any can only be 'static.

Introducing the 'static bound here makes this bound explicit, making
Box<Any> legal.