powerpc: Fix compile warning in init_thread

Currently we get this warning:
arch/powerpc/kernel/init_task.c:33: warning: missing braces around initializer
arch/powerpc/kernel/init_task.c:33: warning: (near initialization for 'init_task.thread.fpr[0]')

This fixes it.

Noticed by Stephen Rothwell.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>