bpo-9566: Fix compiler warnings in pyexpat.c (GH-10654)

Explicit cast a pointer difference (intptr_t) to int to fix
two warnings on 64-bit Windows:

    Modules\pyexpat.c(1181): warning C4244: 'initializing':
    conversion from '__int64' to 'int', possible  loss of data

    Modules\pyexpat.c(1192): warning C4244: 'initializing':
    conversion from '__int64' to 'int', possible  loss of data