Network: Fix IFF_UP conflict on Linux with unity builds

I cannot repro this myself, but from the bug report it seems to be
defined in two headers, so we should consistently use the same one.

Fixes: QTBUG-113787
Pick-to: 6.5
Change-Id: I9f1cc51e379c33dbbf659946340a5207aaeb448a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/network/socket/qnativesocketengine_unix.cpp

@@ -18,7 +18,11 @@
 #include <errno.h>
 #include <fcntl.h>
 #ifndef QT_NO_IPV6IFNAME
+#ifdef Q_OS_LINUX
+#include <linux/if.h>
+#else // Q_OS_LINUX
 #include <net/if.h>
+#endif // !Q_OS_LINUX
 #endif
 #ifdef QT_LINUXBASE
 #include <arpa/inet.h>