Build fix: specify more path to node.h

[dmdeemer]

Building on Ubuntu with Python 3.8, there is a /usr/include/python3.8/node.h, which was being included before ./src/core/node.h.

Resolved the conflict by specifying a complete relative path in manifold-applyops.cc, which appears to be the only place node.h is referenced.

[gsohler]

thank you!