Makefile.extrawarn: Move sign-compare from W=2 to W=3

This -Wsign-compare compiler warning can be very noisy
and most of the suggested conversions are unnecessary.

Make the warning W=3 so it's described under the
"can most likely be ignored" block.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>