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>

include/asm-powerpc/processor.h

@@ -222,7 +222,7 @@ struct thread_struct {
 	.ksp_limit = INIT_SP_LIMIT, \
 	.regs = (struct pt_regs *)INIT_SP - 1, /* XXX bogus, I think */ \
 	.fs = KERNEL_DS, \
-	.fpr = {0}, \
+	.fpr = {{0}}, \
 	.fpscr = { .val = 0, }, \
 	.fpexc_mode = 0, \
 }