libproxy should not be public dependency of Qt5Network

otherwise libs/modules linking against Qt5Network go:

[ 98s] g++ Wl,-no-undefined -shared -Wl,-Bsymbolic-functions -Wl,-soname,libQt5WebSockets.so.5 -o libQt5WebSockets.so.5.5.0 .obj/qwebsocket.o .obj/qwebsocket_p.o .obj/qwebsocketserver.o .obj/qwebsocketserver_p.o .obj/qwebsocketprotocol.o .obj/qwebsockethandshakerequest.o .obj/qwebsockethandshakeresponse.o .obj/qwebsocketdataprocessor.o .obj/qwebsocketcorsauthenticator.o .obj/qwebsocketframe.o .obj/qmaskgenerator.o .obj/qdefaultmaskgenerator_p.o .obj/qsslserver.o .obj/moc_qwebsocket.o .obj/moc_qwebsocketserver.o .obj/moc_qwebsockethandshakeresponse_p.o .obj/moc_qwebsocketdataprocessor_p.o .obj/moc_qsslserver_p.o -L/usr/lib64 -lQt5Network -lproxy -lQt5Core -lpthread
[ 98s] /usr/lib64/gcc/x86_64-suse-linux/4.8/../../../../x86_64-suse-linux/bin/ld: cannot find -lproxy