net/mlx5: fix build with zero-size array

net/mlx5: fix build with zero-size array

If the build environment doesn't have 'linux/tc_act/tc_pedit.h' header,
compiler will use needed structs defined in mlx5_flow_tcf.c.

However, there is a zero-size array defined in one struct and
ISO C forbids this when -Wpedantic is set by debug mode.

Simply put __extension__ keyword before the struct in question.

Fixes: 2ed2fe5f0a9c ("net/mlx5: rewrite IP address UDP/TCP port by E-Switch")
Signed-off-by: Xiaoyu Min <jackmin@mellanox.com>