Fix missing LICENSE file in macros crate

[musicinmybrain]

Add a symlink to the top-level license file to fix the lack of a license file in the published pyo3-async-runtimes-macros crate. Note that the chosen Apache-2.0 license requires its text to be distributed.

Before this PR:

$ cargo publish --dry-run
[…]
$ tar -tzf ../target/package/pyo3-async-runtimes-macros-0.26.0.crate
pyo3-async-runtimes-macros-0.26.0/.cargo_vcs_info.json
pyo3-async-runtimes-macros-0.26.0/Cargo.lock
pyo3-async-runtimes-macros-0.26.0/Cargo.toml
pyo3-async-runtimes-macros-0.26.0/Cargo.toml.orig
pyo3-async-runtimes-macros-0.26.0/README.md
pyo3-async-runtimes-macros-0.26.0/src/lib.rs
pyo3-async-runtimes-macros-0.26.0/src/tokio.rs

After this PR:

$ cargo publish --dry-run
[…]
$ tar -tzf ../target/package/pyo3-async-runtimes-macros-0.26.0.crate
pyo3-async-runtimes-macros-0.26.0/.cargo_vcs_info.json
pyo3-async-runtimes-macros-0.26.0/Cargo.lock
pyo3-async-runtimes-macros-0.26.0/Cargo.toml
pyo3-async-runtimes-macros-0.26.0/Cargo.toml.orig
pyo3-async-runtimes-macros-0.26.0/LICENSE
pyo3-async-runtimes-macros-0.26.0/README.md
pyo3-async-runtimes-macros-0.26.0/src/lib.rs
pyo3-async-runtimes-macros-0.26.0/src/tokio.rs