microblaze: Mark get_frame_size as static

It is used only locally in unwind.c.

The patch removes warning:
arch/microblaze/kernel/unwind.c:62:13: warning: no previous prototype
for 'get_frame_size' [-Wmissing-prototypes]
 inline long get_frame_size(unsigned long instr)

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