Add missing override for adjacent_tokens_only (#745)

Summary:
Resubmitting as https://github.com/facebook/folly/pull/743 failed internal builds according to yfeldblum

Edit: as pointed out in the conversation below, this check needs to be gated for Boost versions [1.59, 1.64] as Boost 1.65 removed this method.
Closes https://github.com/facebook/folly/pull/745

Reviewed By: Orvid

Differential Revision: D6853821

Pulled By: yfeldblum

fbshipit-source-id: 2e0c8f425055ee82c317146c4c836c28f7130e9b

folly/experimental/ProgramOptions.cpp

@@ -82,8 +82,8 @@ class GFlagValueSemanticBase : public po::value_semantic {
     : info_(std::move(info)) { }
 
   std::string name() const override { return "arg"; }
-#if BOOST_VERSION >= 105900
-  bool adjacent_tokens_only() const {
+#if BOOST_VERSION >= 105900 && BOOST_VERSION <= 106400
+  bool adjacent_tokens_only() const override {
     return false;
   }
 #endif