Fix Include Guard in pyAMReX.H (#179)

Every header needs an include guard to avoid duplicate definitions
as this file grows with more content.

Author: ax3l

src/pyAMReX.H

@@ -6,6 +6,8 @@
  * Authors: Axel Huebl
  * License: BSD-3-Clause-LBNL
  */
+#pragma once
+
 #include <pybind11/pybind11.h>
 #include <pybind11/functional.h>
 #include <pybind11/numpy.h>