x11-themes/qtcurve: fix build with GCC-based architectures

[pkubaj]

Summary

GCC9 requires upstream fix because of:
/wrkdirs/usr/ports/x11-themes/qtcurve-gtk2/work/qtcurve-a1ec5f8/.cmake_utils_base/cmake_c_macros/include_fix/qtcurve-utils/gtkprops.h:80:24: error: a reinterpret_cast is not a constant expression

80 |                 return (GObject*)qtcContainerOf(p, Props, name)->m_w;   \
   |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/wrkdirs/usr/ports/x11-themes/qtcurve-gtk2/work/qtcurve-a1ec5f8/.cmake_utils_base/cmake_c_macros/include_fix/qtcurve-utils/gtkprops.h:161:9: note: in expansion of macro 'DEF_WIDGET_SIG_CONN_PROPS'

[tcberner]