net/softnic: fix maybe-uninitialized warning

In pmd_mtr_stats_read() 'struct rte_mtr_stats s' is initialized
during the call to mtr_stats_convert() but on some systems it
is triggering a compiler warning. e.g.

drivers/net/softnic/rte_eth_softnic_meter.c:916:25: error: 's'
   may be used uninitialized [-Werror=maybe-uninitialized]

Initialize the struct in pmd_mtr_stats_read() to avoid warnings.

Reported-by: Ali Alnubani <alialnu@nvidia.com>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>

drivers/net/softnic/rte_eth_softnic_meter.c

@@ -903,7 +903,7 @@ pmd_mtr_stats_read(struct rte_eth_dev *dev,
 
 	/* Stats format conversion. */
 	if (stats || stats_mask) {
-		struct rte_mtr_stats s;
+		struct rte_mtr_stats s = {};
 		uint64_t s_mask = 0;
 
 		mtr_stats_convert(p,