wallet: pass std::function by const ref, not value

Because we can.

src/wallet/wallet2.cpp

@@ -2655,7 +2655,7 @@ uint64_t wallet2::get_upper_tranaction_size_limit()
   return ((full_reward_zone * 125) / 100) - CRYPTONOTE_COINBASE_BLOB_RESERVED_SIZE;
 }
 //----------------------------------------------------------------------------------------------------
-std::vector<size_t> wallet2::select_available_outputs(std::function<bool(const transfer_details &td)> f)
+std::vector<size_t> wallet2::select_available_outputs(const std::function<bool(const transfer_details &td)> &f)
 {
   std::vector<size_t> outputs;
   size_t n = 0;

src/wallet/wallet2.h

@@ -403,7 +403,7 @@ namespace tools
     uint64_t get_upper_tranaction_size_limit();
     void check_pending_txes();
     std::vector<uint64_t> get_unspent_amounts_vector();
-    std::vector<size_t> select_available_outputs(std::function<bool(const transfer_details &td)> f);
+    std::vector<size_t> select_available_outputs(const std::function<bool(const transfer_details &td)> &f);
     std::vector<size_t> select_available_unmixable_outputs(bool trusted_daemon);
 
     cryptonote::account_base m_account;