sch_mqprio: Fix build with older gcc.

  CC [M]  net/sched/sch_mqprio.o
net/sched/sch_mqprio.c: In function ?mqprio_init?:
net/sched/sch_mqprio.c:145: error: unknown field ?tc? specified in initializer
net/sched/sch_mqprio.c:145: warning: missing braces around initializer
net/sched/sch_mqprio.c:145: warning: (near initialization for ?tc.<anonymous>?)
make[2]: *** [net/sched/sch_mqprio.o] Error 1
make[1]: *** [net/sched] Error 2
make: *** [net] Error 2

Several people reported this, surround the unnamed union
member initialization with braces to fix.

Signed-off-by: David S. Miller <davem@davemloft.net>