Fix missing header in setup.py

Author: dean0x7d

setup.py

@@ -28,6 +28,7 @@
         'include/pybind11/embed.h',
         'include/pybind11/eval.h',
         'include/pybind11/functional.h',
+        'include/pybind11/iostream.h',
         'include/pybind11/numpy.h',
         'include/pybind11/operators.h',
         'include/pybind11/options.h',