Export PRODUCT_SOONG_NAMESPACES to Soong

Bug: 65683273
Test: put 'PRODUCT_SOONG_NAMESPACES := jeff-test/dir1' \
      in a .mk file and observe its value copied into \
      out/soong/soong.variables

Change-Id: I2b61caa46f40f35ccecf64da88918b73180e02e8
diff --git a/core/product.mk b/core/product.mk
index c01a856..f15f6b3 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -102,6 +102,7 @@
     PRODUCT_SDK_ADDON_COPY_MODULES \
     PRODUCT_SDK_ADDON_DOC_MODULES \
     PRODUCT_SDK_ADDON_SYS_IMG_SOURCE_PROP \
+    PRODUCT_SOONG_NAMESPACES \
     PRODUCT_DEFAULT_WIFI_CHANNELS \
     PRODUCT_DEFAULT_DEV_CERTIFICATE \
     PRODUCT_RESTRICT_VENDOR_FILES \