common: fix enum redeclaration

common: fix enum redeclaration

This patch fixes the error about enum redeclaration during compile time.

The error is from the enum declaration in src/common/perf_counters.h:

enum unit_t : uint8_t
{
  BYTES,
  NONE
};

When external library also includes similar declaratiron, the following
error will appear:
  error: redeclaration of enumerator 'NONE'
  error: previous definition of 'NONE' ...

Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>