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: NMichal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/unwind.c

@@ -59,7 +59,7 @@ struct stack_trace;
  *
  * Return - Number of stack bytes the instruction reserves or reclaims
  */
-inline long get_frame_size(unsigned long instr)
+static inline long get_frame_size(unsigned long instr)
 {
 	return abs((s16)(instr & 0xFFFF));
 }