Support rust_toolchain_library for rust-project

Bug: 209678277
Test: SOONG_GEN_RUST_PROJECT=1 m nothing; check that libstd is included
Change-Id: Ic197e00bc4695aa6dcd46b8c704a82a2e5e90460
diff --git a/rust/project_json.go b/rust/project_json.go
index ae48312..fe259d6 100644
--- a/rust/project_json.go
+++ b/rust/project_json.go
@@ -211,6 +211,8 @@
 		comp = c.binaryDecorator.baseCompiler
 	case *procMacroDecorator:
 		comp = c.baseCompiler
+	case *toolchainLibraryDecorator:
+		comp = c.baseCompiler
 	default:
 		return nil, nil, false
 	}