trace-cmd: fix ptrace detection

Ptrace support detection is ignored when CFLAGS is set by
buildroot. Use override in Makefile to add -DWARN_NO_PTRACE and
-DNO_PTRACE in CFLAGS if ptrace is not available.

Fixes:
  http://autobuild.buildroot.net/results/60e/60ed317d52d93ba2c087aa6ff4f422d760806d89//



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>