Declare librustrt a static dependency of libextra

This wasn't uncovered during testing because the librustrt is available locally
in the build directory (and it needs to be for all the tests to link against it
as well).

Closes #10765
auto merge of #10770 : alexcrichton/rust/static-librustrt, r=alexcrichton

This wasn't uncovered during testing because the librustrt is available locally
in the build directory (and it needs to be for all the tests to link against it
as well).

Closes #10765