[refactor] move SplitHostPort() into utilstrencodings

This moves SplitHostPort from libbitcoin_common to libbitcoin_util so it
is available to bitcoin-cli.

src/httpserver.cpp

@@ -7,6 +7,7 @@
 #include "chainparamsbase.h"
 #include "compat.h"
 #include "util.h"
+#include "utilstrencodings.h"
 #include "netbase.h"
 #include "rpc/protocol.h" // For HTTP status codes
 #include "sync.h"