plugins: fix -Werror=maybe-uninitialized false-positive

plugins: fix -Werror=maybe-uninitialized false-positive

../contrib/plugins/cache.c:638:9: error: ‘l2_cache’ may be used uninitialized [-Werror=maybe-uninitialized]
  638 |         append_stats_line(rep, l1_dmem_accesses, l1_dmisses,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Is a false-positive, since cores > 1, so the variable is set in the
above loop.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>