Build `_import_macro_dep` in `exec` mode (#1203)

Like the rest of the `proc_macro` dependencies.
diff --git a/rust/private/rust.bzl b/rust/private/rust.bzl
index 3c2c6b5..6a2ed62 100644
--- a/rust/private/rust.bzl
+++ b/rust/private/rust.bzl
@@ -594,6 +594,7 @@
     "_extra_rustc_flags": attr.label(default = "//:extra_rustc_flags"),
     "_import_macro_dep": attr.label(
         default = "@rules_rust//util/import",
+        cfg = "exec",
     ),
     "_process_wrapper": attr.label(
         default = Label("//util/process_wrapper"),