bind: fix compile failure when building path is long

bind: fix compile failure when building path is long

Fix the building path is long, when building bind, we would meet the following
error.

    ".../long/path/to/bind/9.10.2-P3-r0/bind-9.10.2-P3/lib/dns" too long

This is because the in gen.c, DIRNAMESIZE is limited to 256. But in OE, the
path length limit is more than 400. So we change it to 512.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>