net: fix build with -Wpedantic

Adds the __extension__ attribute to resolve the following warning
with gcc 4.8.5 on CentOS 7:
lib/net/rte_macsec.h:38:2: error: type of bit-field 'short_length' is
  a GCC extension [-Werror=pedantic]

Fixes: 0aaf097585 ("ethdev: add MACsec flow item")

Signed-off-by: Ali Alnubani <alialnu@nvidia.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>

lib/net/rte_macsec.h

@@ -28,6 +28,7 @@ extern "C" {
 /**
  * MACsec Header (SecTAG)
  */
+__extension__
 struct rte_macsec_hdr {
 	/**
 	 * Tag control information and Association number of secure channel.