Include Python.h before any glibc header in xla_ffi.cpp

g++ defines _GNU_SOURCE, so glibc 2.44's features.h now defines
_POSIX_C_SOURCE/_XOPEN_SOURCE. xla_ffi.cpp includes c_api.h (-> stdint.h ->
features.h) before py.h/Python.h, so Python's pyconfig.h then redefines
those macros, which the project's -Werror turns into a build failure.

Include Python.h first so pyconfig.h sets the macros before features.h,
which #undefs before redefining them.

--- a/cext/xla_ffi.cpp
+++ b/cext/xla_ffi.cpp
@@ -2,6 +2,8 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
+#include <Python.h>
+
 #ifdef _WIN32
 
 #include "xla/ffi/api/c_api.h"