pw_build: Add mod proc_macro to rust macro targets

Gnaw which translates Cargo.toml to GN does not reads the `lib` section
in Cargo manifest. Some crates puts `proc_macro` in `lib` and does not
specify `use proc_macro` in the source. This change is a workaround for
building those crates.

Change-Id: I1cec9b076931e5175068781cfd3e9c1aefa2f86c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/230013
Commit-Queue: Jiacheng Lu <[email protected]>
Reviewed-by: Erik Gilling <[email protected]>
Lint: Lint 🤖 <[email protected]>

pw_build/rust_proc_macro.gni

@@ -37,6 +37,15 @@ template("pw_rust_proc_macro") {
       pw_toolchain_SCOPE.is_host_toolchain) {
     pw_rust_library(target_name) {
       forward_variables_from(invoker, "*")
+
+      if (!defined(rustflags)) {
+        rustflags = []
+      }
+
+      rustflags += [
+        "--extern",
+        "proc_macro",
+      ]
       underlying_target_type = "rust_proc_macro"
     }
   } else {