test: tst_qhostinfo requires `private_tests' set

Otherwise you get:

  undefined reference to `qt_qhostinfo_clear_cache()'

Change-Id: I32313f290b9e4236440ae01afe12285d4539be60
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>

tests/auto/network/kernel/qhostinfo/qhostinfo.pro

@@ -3,6 +3,7 @@ TARGET = tst_qhostinfo
 
 SOURCES  += tst_qhostinfo.cpp
 
+requires(contains(QT_CONFIG,private_tests))
 QT = core-private network-private testlib
 
 wince*: {