nextpnr: use qtbase-5.11 instead of 5.12

This fixes a bizarre regression in the NextPNR GUI that causes the
background of the OpenGL floorplanning window to become 'transparent' on
certain platforms, greatly limiting its utility. However, QT 5.11 seems
to work just fine here.

Signed-off-by: Austin Seipp <aseipp@pobox.com>