gn2bp: Add rule to replace arg by $(out)

To support //base:build_date

Test: ./update_result.sh, m cronet_aml_base_build_date
Change-Id: Ieacf0d47738f2ca32b3bec5415ac95b02b39c322
diff --git a/tools/gn2bp/update_results.sh b/tools/gn2bp/update_results.sh
index 58f0600..6b46ea2 100755
--- a/tools/gn2bp/update_results.sh
+++ b/tools/gn2bp/update_results.sh
@@ -15,6 +15,7 @@
   "//third_party/zlib:zlib"
   "//third_party/libevent:libevent"
   "//base:base_static"
+  "//base:build_date"
 )
 
 BASEDIR=$(dirname "$0")