folly: fix one more -Wunused-parameter

Summary: The usage is guarded by a preprocessor macro, and I didn't noticed this earlier.

Reviewed By: yfeldblum

Differential Revision: D2887016

fb-gh-sync-id: 791c4d16475aab77235792953997a281354018e9

folly/io/async/AsyncSSLSocket.cpp

@@ -824,6 +824,7 @@ bool AsyncSSLSocket::getSelectedNextProtocolNoThrow(
   }
   return true;
 #else
+  (void)protoType;
   return false;
 #endif
 }