gh-152216: Silence expat compiler warnings

[cdce8p]

Silence expat compiler warnings on macOS with clang.

In file included from ./Modules/expat/xmltok.c:309:
./Modules/expat/xmltok_impl.c:283:5: warning: fallthrough annotation in unreachable code [-Wunreachable-code-fallthrough]
  283 |     CHECK_NMSTRT_CASES(enc, ptr, end, nextTokPtr)
      |     ^
./Modules/expat/xmltok_impl.c:116:5: note: expanded from macro 'CHECK_NMSTRT_CASES'
  116 |     EXPAT_FALLTHROUGH;                                                         \
      |     ^
./Modules/expat/fallthrough.h:45:33: note: expanded from macro 'EXPAT_FALLTHROUGH'
   45 | #      define EXPAT_FALLTHROUGH __attribute__((fallthrough))
      |                                 ^
...

• Issue: Upgrade bundled Expat to 2.8.2 (e.g. for the fix to CVE-2026-56132 and 12 others) #152216

[StanFromIreland]

As I noted in the issue, we won't be doing this.