Fix a missing operator in the internal notice check.

Change-Id: I76a9d8fe3f5af1317a93d9f8976fdb2a2904116a
diff --git a/setup.sh b/setup.sh
index 61f8ae7..7a62cf0 100755
--- a/setup.sh
+++ b/setup.sh
@@ -138,7 +138,7 @@
     fi
 fi
 
-if [[ "${ROOTDIR}/INTERNAL_NOTICE.md" ]]; then
+if [[ -f "${ROOTDIR}/INTERNAL_NOTICE.md" ]]; then
     echo
     echo '*** WARNING: Your repo configuration includes closed-source internal'
     echo '*** repositories. Please watch your step when you contribute code.'