systemc: Push python headers on top of sources

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>