folly: fixup CompressionTest compilation on macos

Summary:
The compiler on macos is pedantic about the size of the 9UL not
matching the size of the type returned by `oneBasedMsbPos`.  Cast it to
the appropriate size.

Reviewed By: meyering

Differential Revision: D3576183

fbshipit-source-id: 41e9afc78eed2994e34238da119774e9bf6b7cea

folly/io/test/CompressionTest.cpp

@@ -213,7 +213,8 @@ void CompressionVarintTest::runSimpleTest(const DataHolder& dh) {
   auto compressed = codec_->compress(original.get());
   auto breakPoint =
       1UL +
-      Random::rand64(std::max(9UL, oneBasedMsbPos(uncompressedLength_)) / 9UL);
+      Random::rand64(
+          std::max(uint64_t(9), oneBasedMsbPos(uncompressedLength_)) / 9UL);
   auto tinyBuf = IOBuf::copyBuffer(compressed->data(),
                                    std::min(compressed->length(), breakPoint));
   compressed->trimStart(breakPoint);