libcutils: 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: Id4f07a9b13a32c563fda6e96e4a87b8b97d125fd
Merge branch 'cm-12.0' of github.com:CyanogenMod/android_system_core into cm-12.0