fix building on Mac

not sure if this is the right way to fix this 
seen here boostorg/process#55 (comment) and mostly here boostorg/process#55 (comment)

Author: 3nids

external/qt-unix-signals/sigwatch.cpp

@@ -108,7 +108,7 @@ void UnixSignalWatcherPrivate::watchForSignal(int signal)
     struct sigaction sigact;
     sigact.sa_handler = UnixSignalWatcherPrivate::signalHandler;
     sigact.sa_flags = 0;
-    ::sigemptyset(&sigact.sa_mask);
+    sigemptyset(&sigact.sa_mask);
     sigact.sa_flags |= SA_RESTART;
     if (::sigaction(signal, &sigact, nullptr)) {
         qDebug() << "UnixSignalWatcher: sigaction: " << ::strerror(errno);