net: fix pedantic build

net: fix pedantic build

when trying to compile rte_mpls with pedantic enabled,
on old compilers like 4.8 it will complain about bit field definition.

error: type of bit-field 'bs' is a GCC extension [-Werror=pedantic]
error: type of bit-field 'tc' is a GCC extension [-Werror=pedantic]
error: type of bit-field 'tag_lsb' is a GCC extension [-Werror=pedantic]

This fixes the compilation error by adding extension to the header
definition.

Fixes: e480cf487a0d ("net: add MPLS header structure")
Cc: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>