microblaze: Fix compilation failure because of release_thread

Warning log:
In file included from arch/microblaze/include/asm/thread_info.h:21:0,
                 from include/linux/thread_info.h:54,
                 from include/asm-generic/preempt.h:4,
                 from arch/microblaze/include/generated/asm/preempt.h:1,
                 from include/linux/preempt.h:18,
                 from include/linux/spinlock.h:50,
                 from include/linux/seqlock.h:35,
                 from include/linux/time.h:5,
                 from include/linux/stat.h:18,
                 from include/linux/sysfs.h:20,
                 from include/linux/kobject.h:21,
                 from include/linux/device.h:17,
                 from include/linux/node.h:17,
                 from include/linux/cpu.h:16,
                 from arch/microblaze/kernel/process.c:11:
arch/microblaze/include/asm/processor.h:125:20: warning: no previous prototype
for 'release_thread' [-Wmissing-prototypes]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>