fix: remove feature requirement for thiscall

[ElonGaties]

abi_thiscall is now considered stable by rust
rust-lang/rust@06daa9e

[Hpmason]

Thanks for helping update this with the recent stabilization!

Though it's stabilized, it's not planned to be released until 1.73.0 on October 5th. So until the official toolchain release, I'd like to keep that under the feature flag, that way current users on stable can still compile.

In order to fix the issue with the nightly feature, the lib.rs file is what actually needs to be changed to remove the cfg_attr for thiscall-abi flag here:

https://github.com/Hpmason/retour-rs/blob/master/src/lib.rs#L11-L14

[ElonGaties]

I have removed that definition, perhaps once thiscall abi can be used on stable this could be pushed.