signals: declare sys_rt_tgsigqueueinfo in syscalls.h

sys_rt_tgsigqueueinfo needs to be declared in linux/syscalls.h so that
architectures defining the system call table in C can reference it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
LKML-Reference: <200907071023.44008.arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>