devel/qt6-tools: unbreak with libglvnd/X11=off

> qt6-tools-6.4.2_1 depends on shared library: libGL.so - not found

> Installing existing package /packages/All/libglvnd-1.6.0.pkg

Installing libglvnd-1.6.0...
Extracting libglvnd-1.6.0: .......... done

> qt6-tools-6.4.2_1 depends on shared library: libGL.so - not found

• Error code 1