metrics: fix compilation with -Og

metrics: fix compilation with -Og

The compilation with gcc-6.3.0 and EXTRA_CFLAGS=-Og gives the following
error:

  CC rte_metrics.o
  rte_metrics.c: In function
  ‘rte_metrics_reg_names’: rte_metrics.c:153:22: error: ‘entry’
                           may be used uninitialized in this function
                           [-Werror=maybe-uninitialized]
    entry->idx_next_set = 0;
    ~~~~~~~~~~~~~~~~~~~~^~~

This is a false positive, gcc is not able to see that we always go in
the loop at least once, initializing entry.

Fix the warning by initializing entry to NULL.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>