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/privet/security_manager.h b/src/privet/security_manager.h
index c99201b..bb513e6 100644
--- a/src/privet/security_manager.h
+++ b/src/privet/security_manager.h
@@ -41,7 +41,7 @@
 
   class KeyExchanger {
    public:
-    virtual ~KeyExchanger() = default;
+    virtual ~KeyExchanger() {}
 
     virtual const std::string& GetMessage() = 0;
     virtual bool ProcessMessage(const std::string& message,