Fixed Bazel test target //src/test/cpp/util:strings_test by adding a missing "using" statement.

--
MOS_MIGRATED_REVID=100262453
diff --git a/src/test/cpp/util/strings_test.cc b/src/test/cpp/util/strings_test.cc
index 1a7c203..54183f4 100644
--- a/src/test/cpp/util/strings_test.cc
+++ b/src/test/cpp/util/strings_test.cc
@@ -14,6 +14,8 @@
 #include "src/main/cpp/util/strings.h"
 #include "gtest/gtest.h"
 
+using std::vector;
+
 namespace blaze_util {
 
 TEST(BlazeUtil, JoinStrings) {