test: fix versionsTest build for old zstd versions

test: fix versionsTest build for old zstd versions

Summary:
Some old zstd versions (notably v0.6.x) have a bug in fileio.c where header includes check for `ZSTD_LEGACY_SUPPORT==1` but code usage checks for `ZSTD_LEGACY_SUPPORT>=1`. Using value 5 causes compilation failure because headers aren't included but the code tries to use legacy functions.

Changing to `ZSTD_LEGACY_SUPPORT=1` for old version builds fixes the compilation while still enabling legacy format support.

Test Plan:
Run `make versionsTest` or `python3 tests/test-zstd-versions.py` to verify all old versions compile and cross-version decompression works correctly.