shamu_defconfig: disable NF_TARGET_REJECT_SKERR

Disable forcing socket error when rejecting with icmp* for now
due to frequent kernel crash.

Bug: 28424847
Change-Id: Id9d45017bf56752d5e027baefbcc7f7e3a52dd08
Signed-off-by: Thierry Strudel <tstrudel@google.com>