net/mlx5: Export modify header alloc/dealloc functions

Those functions will be used by the RDMA side to create modify header
actions to be attached to flow steering rules via verbs.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>