Add std_reduce to c++17 partial support macros.

Author: evoskuil

include/bitcoin/system/preprocessor.hpp

@@ -210,11 +210,13 @@
     #define std_any_of(p, b, e, l) std::any_of((p), (b), (e), (l))
     #define std_all_of(p, b, e, l) std::all_of((p), (b), (e), (l))
     #define std_for_each(p, b, e, l) std::for_each((p), (b), (e), (l))
+    #define std_reduce(p, b, e, i, l) std::reduce((p), (b), (e), (i), (l))
     #define std_transform(p, b, e, t, l) std::transform((p), (b), (e), (t), (l))
 #else
     #define std_any_of(p, b, e, l) std::any_of((b), (e), (l))
     #define std_all_of(p, b, e, l) std::all_of((b), (e), (l))
     #define std_for_each(p, b, e, l) std::for_each((b), (e), (l))
+    #define std_reduce(p, b, e, i, l) std::reduce((b), (e), (i), (l))
     #define std_transform(p, b, e, t, l) std::transform((b), (e), (t), (l))
 #endif