wsd: expose http::Session's socket FD

For logging and informational purposes.

Change-Id: I200a54604c40351516f7947cb2bca8cf01a89c81
Signed-off-by: Ashod Nakashian <ashod.nakashian@collabora.co.uk>

net/HttpRequest.hpp

@@ -1263,6 +1263,8 @@ public:
         }
     }
 
+    /// Returns the socket FD, for logging/informational purposes.
+    int getFD() const { return _fd; }
 
 private:
     inline void logPrefix(std::ostream& os) const { os << '#' << _fd << ": "; }