util: Replace boost::signals2 with std::function

src/wallet/coinselection.cpp

@@ -3,9 +3,12 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <wallet/coinselection.h>
+
 #include <util.h>
 #include <utilmoneystr.h>
 
+#include <boost/optional.hpp>
+
 // Descending order comparator
 struct {
     bool operator()(const OutputGroup& a, const OutputGroup& b) const