Make libziparchive C++-only.

If we're going to build libziparchive with _FILE_OFFSET_BITS=64, which
seems like a reasonable idea, we don't want off_t/off64_t mixups in the
ABI, and C++ name mangling helps protect against that where C wouldn't.

Bug: N/A
Test: builds
Change-Id: I5f068b41fb4cd9e1e055e5250054318b35bcbc08