Fix GCC 4.7 errors

GCC 4.7.2 fail overriding constructors defined with virtual and default.
Debian build also fails linking  without -lrt.
INT64_C is not defined on Debian with 4.7.2. "ll" should be enough.

Change-Id: Ia779f094ed64fc3b1941429485e81520d017c2f5
Reviewed-on: https://weave-review.googlesource.com/1671
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
diff --git a/src/notification/notification_channel.h b/src/notification/notification_channel.h
index 5fb7993..ef152a8 100644
--- a/src/notification/notification_channel.h
+++ b/src/notification/notification_channel.h
@@ -17,7 +17,7 @@
 
 class NotificationChannel {
  public:
-  virtual ~NotificationChannel() = default;
+  virtual ~NotificationChannel() {}
 
   virtual std::string GetName() const = 0;
   virtual bool IsConnected() const = 0;