folly: fixup CompressionTest compilation on macos
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

Orvid/folly,mqeizi/folly,floxard/folly,Orvid/folly,facebook/folly,floxard/folly,floxard/folly,rklabs/folly,facebook/folly,charsyam/folly,Orvid/folly,floxard/folly,rklabs/folly,facebook/folly,charsyam/folly,rklabs/folly,facebook/folly,rklabs/folly,mqeizi/folly,mqeizi/folly,mqeizi/folly,floxard/folly,rklabs/folly,charsyam/folly,charsyam/folly,Orvid/folly,facebook/folly,Orvid/folly,mqeizi/folly