Declare raise() for Mac OS X.

This prevents the warning, "implicit declaration of function raise",
in programs that call raise().  I forgot to declare it because the
function raise() is in libc but the declaration goes in libsys.

plat/osx/include/unistd.h

@@ -120,6 +120,8 @@ int kill(pid_t, int);
 int sigaction(int, const struct sigaction *, struct sigaction *);
 sig_t signal(int, sig_t);
 
+int raise(int); /* in libc */
+
 /* XXX - end signal.h */
 
 void _exit(int);