toolbox: make "r" available for /vendor

Make "r" package available for /vendor, so that we can easily add:

PRODUCT_PACKAGES_DEBUG += r.vendor

in device/google/<device>/device.mk

Change-Id: I46c23a85c95444083b8e4b68961c541d9af9b216
diff --git a/toolbox/Android.bp b/toolbox/Android.bp
index f08cf93..7ad6f1c 100644
--- a/toolbox/Android.bp
+++ b/toolbox/Android.bp
@@ -59,6 +59,7 @@
     name: "r",
     defaults: ["toolbox_defaults"],
     srcs: ["r.c"],
+    vendor_available: true,
 }
 
 // We build BSD grep separately (but see http://b/111849261).