Flag libproxy as a private library

libproxy is not part of Qt's public interface, but a helper library
used by Qt. Treat it as such by using LIB_PRIVATE.

Task-number: QTBUG-44326
Change-Id: I014f992e4b7a51de1e5a7407ce1bd08dd5352136
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/network/kernel/kernel.pri

@@ -60,6 +60,6 @@ else:blackberry {
 }
 else:contains(QT_CONFIG, libproxy) {
     SOURCES += kernel/qnetworkproxy_libproxy.cpp
-    LIBS += -lproxy
+    LIBS_PRIVATE += -lproxy
 }
 else:SOURCES += kernel/qnetworkproxy_generic.cpp