Fails to compile with dev-libs/libsigc++-3.4.0 due to deprecation of track_obj(), now track_object() in dev-libs/libsigc++

Steps to reproduce:

build failed due to deprecation of track_obj(), now track_object() in dev-libs/libsigc++

Build fails with:

src/ui/dialog/color-item.cpp: In constructor ‘Inkscape::UI::Dialog::ColorItem::ColorItem(SPGradient*, Inkscape::UI::Dialog::DialogBase*)’:
src/ui/dialog/color-item.cpp:97:36: error: ‘track_obj’ is not a member of ‘sigc’; did you mean ‘track_object’?

It was deprecated in dev-libs/libsigc++-3.4.0 https://github.com/libsigcplusplus/libsigcplusplus/blob/master/NEWS#L3

Inkscape Version and Operating System:

• Inkscape Version: latest git sources
• Operating System: Gentoo (https://bugs.gentoo.org/892283)
• Operating System version: ~amd64