[master] Trivial fix for the compilation error on RHEL.

This fix was reviewed and approved on Jabber.

src/bin/d2/io_service_signal.h

@@ -20,6 +20,7 @@
 #include <exceptions/exceptions.h>
 
 #include <map>
+#include <stdint.h>
 
 namespace isc {
 namespace d2 {