systemc: Push python headers on top of sources

Some build failures has been seen after USE_SYSTEMC being True by
default and that has been caused by double definition of _XOPEN_SOURCE
and _POSIX_C_SOURCE in some python versions (like 2.7.5, 2.7.13) and
/usr/include/features.h (used by gcc)

Python definition should preceed features.h one, since the latter will
manually #undef them before #define them.

Change-Id: I774711aaf8145df9ad7677a393a60cf3662d6816
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14095
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/core/sc_main_python.cc

@@ -27,12 +27,14 @@
  * Authors: Gabe Black
  */
 
+// This should be on top since it is including python headers
+#include "systemc/core/python.hh"
+
 #include <cstring>
 #include <string>
 
 #include "base/fiber.hh"
 #include "base/logging.hh"
-#include "systemc/core/python.hh"
 #include "systemc/core/sc_main_fiber.hh"
 
 namespace