trace: fix string conversion error on python 2.7.5

On python 2.7.5 python complains about struct() not accepting unicode
argument. Fix by adding explicit conversion.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

scripts/osv/trace.py

@@ -162,7 +162,7 @@ def split_format(format_str):
             fmt = str(c)
             while c.isdigit():
                 c = next(chars)
-                fmt += c
+                fmt += str(c)
             yield fmt
 
 formatters = {