[LyX/master] Shut up compiler warning

[Georg Baum]

commit f728257f264f4b5591b4d4abfdd3d69e7460574b
Author: Georg Baum <b...@lyx.org>
Date:   Sat May 16 14:05:29 2015 +0200

    Shut up compiler warning

diff --git a/boost/boost/optional/optional.hpp 
b/boost/boost/optional/optional.hpp
index afcb807..64ddac8 100644
--- a/boost/boost/optional/optional.hpp
+++ b/boost/boost/optional/optional.hpp
@@ -1251,7 +1251,7 @@ get_pointer ( optional<T>& opt )
 // The following declaration prevents a bug where operator safe-bool is used 
upon streaming optional object if you forget the IO header.
 template<class CharType, class CharTrait>
 std::basic_ostream<CharType, CharTrait>&
-operator<<(std::basic_ostream<CharType, CharTrait>& out, 
optional_detail::optional_tag const& v)
+operator<<(std::basic_ostream<CharType, CharTrait>&, 
optional_detail::optional_tag const&)
 {
   BOOST_STATIC_ASSERT_MSG(sizeof(CharType) == 0, "If you want to output 
boost::optional, include header <boost/optional/optional_io.hpp>"); 
 }