perf test: Fix build of BPF and LLVM on older glibc libraries

perf test: Fix build of BPF and LLVM on older glibc libraries

  $ rpm -q glibc
  glibc-2.12-1.166.el6_7.1.x86_64

<SNIP>
    CC       /tmp/build/perf/tests/llvm.o
  cc1: warnings being treated as errors
  tests/llvm.c: In function ‘test_llvm__fetch_bpf_obj’:
  tests/llvm.c:53: error: declaration of ‘index’ shadows a global declaration
  /usr/include/string.h:489: error: shadowed declaration is here
<SNIP>
    CC       /tmp/build/perf/tests/bpf.o
  cc1: warnings being treated as errors
  tests/bpf.c: In function ‘__test__bpf’:
  tests/bpf.c:149: error: declaration of ‘index’ shadows a global declaration
  /usr/include/string.h:489: error: shadowed declaration is here
<SNIP>

Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: pi3orama@163.com
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Zefan Li <lizefan@huawei.com>
Fixes: b31de018a628 ("perf test: Enhance the LLVM test: update basic BPF test program")
Fixes: ba1fae431e74 ("perf test: Add 'perf test BPF'")
Link: http://lkml.kernel.org/n/tip-akpo4r750oya2phxoh9e3447@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>