Cleanup to DECLARE_string in header file, and make sure dependency is clearer.

Change-Id: I103575ef6d56a3f50da0c60ca44aacb3f02194e9
diff --git a/aos/network/team_number.h b/aos/network/team_number.h
index 91a61f9..c2954b6 100644
--- a/aos/network/team_number.h
+++ b/aos/network/team_number.h
@@ -6,6 +6,10 @@
 #include <optional>
 #include <string>
 
+#include "glog/logging.h"
+
+DECLARE_string(override_hostname);
+
 namespace aos {
 namespace network {