net/i40e: add C++ include guard

Add extern "C" in rte_pmd_i40e.h when be compiled with CPP.

Fixes: 17e906a1ea ("net/i40e: support link status notification")
Cc: stable@dpdk.org

Signed-off-by: Prateek Agarwal <prateekag@cse.iitb.ac.in>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>

drivers/net/i40e/rte_pmd_i40e.h

@@ -14,6 +14,10 @@
  *
  */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <rte_compat.h>
 #include <rte_ethdev.h>
 #include <rte_ether.h>
@@ -1130,4 +1134,8 @@ __rte_experimental
 int
 rte_pmd_i40e_set_switch_dev(uint16_t port_id, struct rte_eth_dev *switch_dev);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _PMD_I40E_H_ */