linuxthreads: add back signal.h

Somehow this got removed with f1d7505e40654a185843bdc8f1cf1fd00ab55c04.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

libpthread/linuxthreads/sysdeps/pthread/pthread.h

@@ -20,6 +20,7 @@
 #include <sched.h>
 #include <time.h>
 
+#include <signal.h>
 #include <bits/pthreadtypes.h>
 #include <bits/initspin.h>