Add PRODUCT_IOT to the list of product vars.

This variable tells whether the product is an Android Things variant of
Android.

Bug: 32549448
Test: `make dist` on an iot product.
Change-Id: I0875951ae57867bc8bf47885e77e494d6284acdb
diff --git a/core/product.mk b/core/product.mk
index 7d7c68b..fffd7c8 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -128,6 +128,7 @@
     VENDOR_EXCEPTION_MODULES \
     VENDOR_EXCEPTION_PATHS \
     PRODUCT_ART_USE_READ_BARRIER \
+    PRODUCT_IOT \