Add missing thread include

This is required for OpenBSD.

Authored by Ryan in bitcoin/bitcoin#33219 (comment)

Author: marco

src/mp/util.cpp

@@ -16,6 +16,7 @@
 #include <sys/socket.h>
 #include <sys/wait.h>
 #include <system_error>
+#include <thread>
 #include <unistd.h>
 #include <utility>
 #include <vector>