libsysutils: Do not use the kernel headers

No idea why this is here, but it breaks arm64 builds. bionic's signal.h
must be used on those

Change-Id: I64f71565b179af38345fcf1496274ee5858abb05
Merge branch 'cm' into HEAD
Merge branch 'L5' of https://github.com/VanirAOSP/system_core into HEAD