lib: os: cbprintf: fix different signedness comparison

One of the loop is using int as loop variable but it's being
compared against size_t. Some compiler configuration has
-Wsign-compare turned on by default which will complain about
"comparison of integer expressions of different signedness".
So fix it by changing the loop variable to be size_t also.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung