Fix constants being created constantly

That code isn't intended to be realtime, so running it every loop
iteration is a bad idea.

Change-Id: Id6e00fc2eedfb50caa56bce9416c11b26a3d3cd7
diff --git a/y2018/constants.cc b/y2018/constants.cc
index 90e4441..833353d 100644
--- a/y2018/constants.cc
+++ b/y2018/constants.cc
@@ -138,7 +138,10 @@
 
 }  // namespace
 
-const Values &GetValues() { return DoGetValues(); }
+const Values &GetValues() {
+  static const Values r = DoGetValues();
+  return r;
+}
 
 const Values &GetValuesForTeam(uint16_t team_number) {
   static ::aos::Mutex mutex;