Compile with clang's libc++

When using clang's libc++, unistd.h is not pulled in automatically
anymore (like libstdc++ does). Include unistd.h on UNIX explicitly
instead.

Change-Id: I21dae6ab2446aca258877e9a2ebe1f7d0a4ef59f
Reviewed-by: Konstantin Tokarev <annulen@yandex.ru>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/libs/zeroconf/servicebrowser.cpp

@@ -46,6 +46,10 @@
 #include <errno.h>
 #include <limits>
 #include <signal.h>
+#ifdef Q_OS_UNIX
+// for select()
+#  include <unistd.h>
+#endif
 
 #include <QAtomicPointer>
 #include <QCoreApplication>