Fix ffi-libponyc-standalone link failure on OpenBSD

PR #5026 fixed the identical missing zlib issue in pony-doc but
missed this test, which has the same `use "lib:z" if not (windows
or openbsd)` guard. LLVM's lld references zlib symbols (adler32,
adler32_combine) for ELF section compression, so OpenBSD needs
explicit -lz just like other POSIX platforms.

Author: SeanTAllen

test/full-program-tests/ffi-libponyc-standalone/main.pony

@@ -1,5 +1,5 @@
 use "lib:ponyc-standalone"
-use "lib:z" if not (windows or openbsd)
+use "lib:z" if not windows
 use "lib:c++" if osx
 
 use @token_new[NullablePointer[TokenStub]](token_id: TokenId)