LibC: Fix typo in signal.h: ol_dset -> old_set

Userland/Libraries/LibC/signal.h

@@ -44,7 +44,7 @@ struct sigaction {
 int kill(pid_t, int sig);
 int killpg(int pgrp, int sig);
 sighandler_t signal(int sig, sighandler_t);
-int pthread_sigmask(int how, const sigset_t* set, sigset_t* ol_dset);
+int pthread_sigmask(int how, const sigset_t* set, sigset_t* old_set);
 int sigaction(int sig, const struct sigaction* act, struct sigaction* old_act);
 int sigemptyset(sigset_t*);
 int sigfillset(sigset_t*);