Clean up things that include script/standard.h

Remove standard.h from files that don't use anything in it, and include
it in files that do.

src/key_io.cpp

@@ -6,6 +6,7 @@
 
 #include <base58.h>
 #include <bech32.h>
+#include <script/standard.h>
 #include <util/strencodings.h>
 
 #include <algorithm>