Add numpy include dir when building

setup.py

 from setuptools import Extension, setup
 from Cython.Build import cythonize
 
+import numpy
+
 extensions = [
     Extension("pycrystfel.bindings", ["pycrystfel/bindings.pyx"],
         libraries=['crystfel'],
+        include_dirs=[numpy.get_include()],
     )
 ]
 setup(