rpc: add missing string_view include to server.h

CRPCTable::help() takes std::string_view but server.h relies on
transitive includes for it.

Add the direct include (probably makes iwyu happier too?)

src/rpc/server.h

@@ -13,6 +13,7 @@
 #include <functional>
 #include <map>
 #include <string>
+#include <string_view>
 
 #include <univalue.h>