minor cmake change as per #306

CMakeLists.txt

@@ -57,7 +57,7 @@ function(pybind11_add_module target_name)
   add_library(${target_name} MODULE ${ARGN})
   target_include_directories(${target_name}
     PRIVATE ${PYBIND11_INCLUDE_DIR}
-    PUBLIC ${PYTHON_INCLUDE_DIRS})
+    PRIVATE ${PYTHON_INCLUDE_DIRS})
 
   # The prefix and extension are provided by FindPythonLibsNew.cmake
   set_target_properties(${target_name} PROPERTIES PREFIX "${PYTHON_MODULE_PREFIX}")