fix(core): include <string> for PerfLog's job hooks

jobQueue, jobStart and jobFinish take their name as std::string const&, so this
header has to provide <string> itself. The rpc hooks moved to string_view and
the include went with them, which include-cleaner reports as an error.

include/xrpl/core/PerfLog.h

@@ -11,6 +11,7 @@
 #include <functional>
 #include <memory>
 #include <span>
+#include <string>
 #include <string_view>
 
 namespace beast {