This makes rust_hawktracer_sys build on debug mode

I am not sure when this triggered and if it's a proc macro issue or what's going on, but I recently noticed that this crate triggers a build of it's dependency `rust_hawktracer_sys` in debug mode :(, even if the crates dependingon rust_hawktracer build in release.

This is not only super bad, as having a profiling library in debug mode is kind of pointless, but also it breaks at build time as on windows the linker tries to search the RelWithDebInfo path of the build folder but that's not existing.