librt: re-add SIGCANCEL to the list of blocked signal in helper thread

Indeed if the libpthread is before the libc in the library look up
the SIGCANCEL is removed from the list of the blocked signal by
sigfillset func, this can produce the handler not properly called.
This commit revert what Denys modified in commit
162cfaea20d807f0ae329efe39292a9b22593b41.

Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

libpthread/nptl/sysdeps/unix/sysv/linux/timer_routines.c

@@ -175,7 +175,7 @@ __start_helper_thread (void)
   sigset_t ss;
   sigset_t oss;
   sigfillset (&ss);
-  /*__sigaddset (&ss, SIGCANCEL); - already done by sigfillset */
+  __sigaddset (&ss, SIGCANCEL);
   INTERNAL_SYSCALL_DECL (err);
   INTERNAL_SYSCALL (rt_sigprocmask, err, 4, SIG_SETMASK, &ss, &oss, _NSIG / 8);