hdata/i2c.c: fix building with gcc8
hdata/i2c.c: fix building with gcc8

hdata/test/../i2c.c:200:1: error: alignment 1 of ‘struct host_i2c_hdr’ is less than 4 [-Werror=packed-not-aligned]
 } __packed;
 ^

Fixes: https://github.com/open-power/skiboot/issues/160
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

legoater/skiboot,qemu/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,shenki/skiboot,open-power/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,stewart-ibm/skiboot,open-power/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot