Use Boost hash_fwd.hpp include instead of hash.hpp directly

Boost recommends this as the latter includes a whole bunch of other headers.

src/image.cpp

@@ -42,7 +42,7 @@
 #include "utils/functional.hpp"
 
 #include <boost/algorithm/string.hpp>
-#include <boost/functional/hash.hpp>
+#include <boost/functional/hash_fwd.hpp>
 
 #include <set>
 

src/map/location.cpp

@@ -27,7 +27,7 @@
 #include "log.hpp"
 #include "utils/math.hpp"
 
-#include <boost/functional/hash.hpp>
+#include <boost/functional/hash_fwd.hpp>
 
 static lg::log_domain log_config("config");
 #define ERR_CF LOG_STREAM(err, log_config)

src/tstring.cpp

@@ -25,7 +25,7 @@
 #include "tstring.hpp"
 #include "gettext.hpp"
 #include "log.hpp"
-#include <boost/functional/hash.hpp>
+#include <boost/functional/hash_fwd.hpp>
 
 #include <boost/multi_index_container.hpp>
 #include <boost/multi_index/hashed_index.hpp>