perf tools: Fix build on older systems

In RHEL 6.7:

  CC       /tmp/build/perf/util/parse-events.o
  cc1: warnings being treated as errors
  util/parse-events.c: In function ‘parse_events_add_cache’:
  util/parse-events.c:366: error: declaration of ‘error’ shadows a global declaration
  util/util.h:136: error: shadowed declaration is here

Rename it to 'err'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 43d0b978 ("perf tools: Enable config and setting names for legacy cache events")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>