Add `#[used]` to `LINKME_PLEASE` static.

[azriel91]

Hiya, when compiling the tests in release mode on Ubuntu 19.04, linking would fail with the following:

• ld (/usr/bin/x86_64-linux-gnu-ld)

  ..(redacted).. undefined reference to `__stop_linkme_EMPTY'
  ..(redacted).. undefined reference to `__start_linkme_EMPTY'

• lld

  ld: error: undefined symbol: __stop_linkme_EMPTY
        >>> referenced by distributed_slice.11cifgtw-cgu.5
        ..(redacted)..
  ld: error: undefined symbol: __start_linkme_EMPTY
        >>> referenced by distributed_slice.11cifgtw-cgu.5
        ..(redacted)..

Adding the #[used] annotation prevents the symbols from being optimized out and is tested to work with both ld and lld. Credit to @kabergstrom.

I'm using cargo defaults -- haven't tweaked optimization settings.

[dtolnay]

Published in 0.1.3.