add gdb to default set

Change-Id: I28f9187735d49048e0504d70832bd9d5b03f6bc9
diff --git a/update.sh b/update.sh
index 7b9a043..da7e819 100755
--- a/update.sh
+++ b/update.sh
@@ -118,6 +118,8 @@
   download_go
   download_godepfile
   download_qemu
+  # See IN-29. Need to distinguish bots from humans.
+  download_gdb
 }
 
 function echo_error() {