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-12.0' of github.com:CyanogenMod/android_system_core into cm-12.0