fix getopt_long compilation with VS 2017
fix getopt_long compilation with VS 2017

Summary:
The compiler is rightly erroring out when we're trying
to assign a const string literal to the non-const `optarg` global.

We "can't" change the definition for `optarg` because it is part
of the getopt interface, so we need some casts to make the compiler
happier.

Refs: https://github.com/facebook/watchman/pull/636

Reviewed By: simpkins

Differential Revision: D9396874

fbshipit-source-id: d777fcd1791e8792fc9c33486b0e251854b3bce9

nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,wez/watchman,wez/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,wez/watchman,wez/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,wez/watchman,facebook/watchman,wez/watchman,wez/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,wez/watchman