Add sysroot property to Darwin prebuilts.

Bug: 169112496
Bug: 168729404
Test: Darwin build finds correct variant dependencies when building.
Change-Id: I1dd0c1dbfbc028efc257ebe4043f99d2740f5ed7
diff --git a/darwin-x86/Android.bp b/darwin-x86/Android.bp
index e778b15..24c38bf 100644
--- a/darwin-x86/Android.bp
+++ b/darwin-x86/Android.bp
@@ -17,6 +17,7 @@
     },
     enabled: false,
     host_supported: true,
+    sysroot: true,
 }
 
 rust_prebuilt_library {
@@ -38,4 +39,5 @@
     },
     enabled: false,
     host_supported: true,
+    sysroot: true,
 }