Don't build the async signal handler test on Windows
Don't build the async signal handler test on Windows

Summary: `AsyncSignalHandler` is not currently supported on Windows, so we shouldn't be attempting to build it.

Reviewed By: yfeldblum

Differential Revision: D5437276

fbshipit-source-id: b3f281bfc501444bb22056860ee785a2beb9b671

Orvid/folly,facebook/folly,rklabs/folly,facebook/folly,Orvid/folly,Orvid/folly,rklabs/folly,rklabs/folly,Orvid/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,Orvid/folly