src/libpcp_qwt/src/qwt_point_3d.h: add explicit declaration for = operator
src/libpcp_qwt/src/qwt_point_3d.h: add explicit declaration for = operator

Was generating warnings on Fedora 30.

Change follows suggestion from nkshirsa@redhat.com (thanks Nikhil).

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp