sudo: disable SSP for auxiliary build tools mksiglist and mksigname

sudo: disable SSP for auxiliary build tools mksiglist and mksigname

The do_compile_prepend() fragment ensures that the non-installable build
tools mksiglist and mksigname are built using the BUILD_CC, but if the the
BUILD_CC does not support SSP and the cross compiler does, the build fails
due to the SSP flags set in the Makefile. Ensuring that SSP is not enabled
when building these tools prevents this from happening.

(From OE-Core rev: ad9a9c6e6b4f00bc47449bbe8c85777ea7c89b88)

Signed-off-by: Tomas Frydrych <tomas@sleepfive.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>