Fix compile error C2664 on win32

[cgohlke]

Fixes compile errors on win32 (32-bit Windows binaries):

    src/csrc/conpty_common.cpp(40): error C2664: 'BOOL InitializeProcThreadAttributeList(LPPROC_THREAD_ATTRIBUTE_LIST,DWORD,DWORD,PSIZE_T)': cannot convert argument 4 from 'size_t *' to 'PSIZE_T'
    src/csrc/conpty_common.cpp(40): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
    C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\um\processthreadsapi.h(673): note: see declaration of 'InitializeProcThreadAttributeList'
    src/csrc/conpty_common.cpp(50): error C2664: 'BOOL InitializeProcThreadAttributeList(LPPROC_THREAD_ATTRIBUTE_LIST,DWORD,DWORD,PSIZE_T)': cannot convert argument 4 from 'size_t *' to 'PSIZE_T'
    src/csrc/conpty_common.cpp(50): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
    C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\um\processthreadsapi.h(673): note: see declaration of 'InitializeProcThreadAttributeList'

[andfoy]

Thanks for this contribution @cgohlke!