net/mlx5e: fix mlx5e_tc_tun_update_header_ipv6 dummy definition

The alternative implementation of this function in a header file
is declared as a global symbol, and gets added to every .c file
that includes it, which leads to a link error:

arm-linux-gnueabi-ld: drivers/net/ethernet/mellanox/mlx5/core/en_rx.o: in function `mlx5e_tc_tun_update_header_ipv6':
en_rx.c:(.text+0x0): multiple definition of `mlx5e_tc_tun_update_header_ipv6'; drivers/net/ethernet/mellanox/mlx5/core/en_main.o:en_main.c:(.text+0x0): first defined here

Mark it 'static inline' like the other functions here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>