Fixes build, which has been broken for all platforms (except BSD?) since
grandparent 406d9ba87b9f6e57b86c6282bf157e3341aa195c.
Thanks to @SmallJoker for pointing out a solution.
Build broke since last commit 479f38973e13680d6a39d9c2a7f29fd330b67d41 for compilers not supporting C++11.
Pre C++11, the constructor of std::ifstream only allows C like strings.
NodeResolver name lists now belong to the NodeResolver object instead of
the associated NodeDefManager. In addition to minimizing unnecessary
abstraction and overhead, this move permits NodeResolvers to look up nodes
that they had previously set pending for resolution. So far, this
functionality has been used in the case of schematics for
serialization/deserialization.