tools/perf/build: Improve the 'stackprotector' feature test

Namhyung Kim noticed that the stackprotector testcase was incomplete:

  > The flag being checked should be -"W"stack-protector instead of
  > -"f"stack-protector.  And the gcc manpage says that -Wstack-protector is
  > only active when -fstack-protector is active.  So the end result should
  > look like
  >
  >         $(BUILD) -Werror -fstack-protector -Wstack-protector

Add -Wstack-protector.

Reported-by: Namhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>