NodesWalk.h: Fix '-Wmissing-field-initializers' warning
NodesWalk.h: Fix '-Wmissing-field-initializers' warning

Fixes:

[  8%] Building CXX object tests/CMakeFiles/bitvector-test.dir/bitvector-test.cpp.o
In file included from /dg/tests/nodes-walk-test.cpp:4:0:
/dg/include/dg/NodesWalk.h: In constructor ‘dg::NodesWalk<Node, Queue, VisitTracker, EdgeChooser>::NodesWalk() [with Node = Node; Queue = dg::ADT::QueueLIFO<Node*>; VisitTracker = dg::SetVisitTracker<Node>; EdgeChooser = dg::SuccessorsEdgeChooser<Node>]’:
/dg/include/dg/NodesWalk.h:109:5: warning: missing initializer for member ‘dg::SetVisitTracker<Node>::_visited’ [-Wmissing-field-initializers]
     NodesWalk() = default;
     ^
...

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg