Make proto-data-store.h depends on absl/strings/str_cat.h directly

protostore/proto-data-store.h

diff --git a/protostore/proto-data-store.h b/protostore/proto-data-store.h
index 670d2be..8c99d55 100644
--- a/protostore/proto-data-store.h
+++ b/protostore/proto-data-store.h
@@ -23,6 +23,7 @@
 #include "absl/base/thread_annotations.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
+#include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
 #include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"