devel/sfml: unbreak build with Clang 6 (C++14 by default)

src/SFML/Window/Unix/WindowImplX11.cpp:1278:36: error: reinterpret_cast from 'nullptr_t' to 'void *' is not allowed

reinterpret_cast<void*>(NULL));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported by: pkg-fallout (blocks 3 ports)
Approved by: portmgr blanket