fix uninitialized timeval struct in metrics_hub

This was causing issue in some builds done on the Ubuntu
PPA systems leading to an SIGILL when starting the UE/eNB

lib/include/srslte/common/metrics_hub.h

@@ -35,8 +35,11 @@ class metrics_hub : public periodic_thread
 public:
   metrics_hub()
     :m(NULL)
-    ,sleep_period_start()
+  {
-  {}
+    for (int i = 0; i < 3; ++i) {
+      sleep_period_start[i] = (struct timeval){};
+    }
+  }
   bool init(metrics_interface<metrics_t> *m_, float report_period_secs_=1.0) {
     m = m_;
     // Start with user-default priority