Enable libdexfile_external_headers on mac.

It was disabled when it started using art_defaults in
https://r.android.com/1875689, but libdexfile depends on it and is
enabled on mac.

Test: Heavy presubmit
Bug: 205747790
Bug: 204364566
Change-Id: I17428034ef8db06eb7084596117d7982132085fb
diff --git a/libdexfile/Android.bp b/libdexfile/Android.bp
index 34fbb14..2737676 100644
--- a/libdexfile/Android.bp
+++ b/libdexfile/Android.bp
@@ -325,6 +325,9 @@
         windows: {
             enabled: true,
         },
+        darwin: {
+            enabled: true,
+        },
     },
 
     apex_available: [