ospfd: Fix compile warning with `-Wformat-truncation`

ospfd: Fix compile warning with `-Wformat-truncation`

ospfd/ospf_sr.c: In function ‘show_sr_node.part.5’:
ospfd/ospf_sr.c:2745:32: warning: ‘%u’ directive output may be truncated writing between 1 and 10 bytes into a region of size 2 [-Wformat-truncation=]
    snprintf(tmp, sizeof(tmp), "%u", i);
                                ^~
ospfd/ospf_sr.c:2745:31: note: directive argument in the range [0, 2147483646]
    snprintf(tmp, sizeof(tmp), "%u", i);

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>