add missing prerequisite

setup.py

@@ -147,6 +147,7 @@ setup(name='pyHealpix',
       author='Martin Reinecke',
       author_email='martin@mpa-garching.mpg.de',
       packages=[],
+      setup_requires={'pybind11': 'pybind11>=2.2.1'},
       extras_require={'numpy': 'numpy>=1.10.4', 'pybind11': 'pybind11>=2.2.1'},
       ext_modules=get_extension_modules(),
       install_requires=['pybind11>=2.2.1']