add gcc 7.x support

Add some inclusion to <functional> Std C++ header to support
build on GCC 7.x.

Change-Id: I82ae10a300807cb6b3cab172bf5d7399264cd08e
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>

src/account_factory.h

@@ -27,6 +27,7 @@
 #include <memory>
 #include <mutex>
 #include <utility>
+#include <functional>
 
 namespace ring {
 

src/call.h

@@ -40,6 +40,7 @@
 #include <condition_variable>
 #include <set>
 #include <list>
+#include <functional>
 
 namespace ring {
 

src/sip/sipvoiplink.h

@@ -46,6 +46,7 @@
 #include <map>
 #include <mutex>
 #include <memory>
+#include <functional>
 
 namespace ring {
 

src/thread_pool.h

@@ -23,6 +23,7 @@
 #include <vector>
 #include <queue>
 #include <future>
+#include <functional>
 
 namespace ring {