ptrace: Fix build problems with glibc-2.18

Since glibc-2.18, the struct ptrace_peeksiginfo_args is
also defined in sys/ptrace.h and conflicts with the definition
from the Linux kernel in linux/ptrace.h. We fix this by not
including the linux/ptrace.h header if the sys/ptrace.h header
defines the said struct. Fixes
https://github.com/linux-test-project/ltp/issues/6

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>