tools lib bpf: Fix compiler warning on CentOS 6

tools lib bpf: Fix compiler warning on CentOS 6

    CC       libbpf.o
  cc1: warnings being treated as errors
  libbpf.c: In function 'bpf_program__title':
  libbpf.c:1037: error: declaration of 'dup' shadows a global declaration
  /usr/include/unistd.h:528: error: shadowed declaration is here
  mv: cannot stat `./.libbpf.o.tmp': No such file or directory
  make[3]: *** [libbpf.o] Error 1
  make[2]: *** [libbpf-in.o] Error 2
  make[1]: *** [/linux/tools/lib/bpf/libbpf.a] Error 2
  make[1]: *** Waiting for unfinished jobs....

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1446549665-2342-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>