.cargo: add missing build section for MacOS

See https://zshipko.github.io/ocaml-rs/\#initial-setup

Author: dannywillems

.cargo/config.toml

@@ -1,2 +1,8 @@
 [alias]
 xtask = "run --package xtask --"
+
+# See https://zshipko.github.io/ocaml-rs/#initial-setup
+# Required to avoid undefined symbols when building dylib on MacOS
+# (e.g. required for building plonk-wasm/plonk-neon at the time of writing).
+[build]
+rustflags = ["-C", "link-args=-Wl,-undefined,dynamic_lookup"]