bpo-30604: Fix __PyCodeExtraState_Get() prototype (#2152)

bpo-30604: Fix __PyCodeExtraState_Get() prototype (#2152)

Replace __PyCodeExtraState_Get() with __PyCodeExtraState_Get(void) to
fix the following GCC warning:

./Include/pystate.h:63:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
 __PyCodeExtraState* __PyCodeExtraState_Get();