abseil-cpp now depends on `pthread.h`, it previously didn't. But this appears to
be a bug in abseil of sorts, because if you have `ABSL_FORCE_THREAD_IDENTITY_MODE` set
to `ABSL_THREAD_IDENTITY_MODE_USE_CPP11` it doesn't appear to be used/needed.
[Zig doesn't ship with pthread for the MinGW / `x86_64-windows-gnu` target](https://github.com/ziglang/zig/issues/10989),
and so the header is missing - but we don't actually need it, so we just add an
empty file to prevent the missing include error.
Signed-off-by: Stephen Gutekanst <stephen@hexops.com>
