net/mlxfw: select CONFIG_XZ_DEC

The new mlxfw code fails to build without the xz library:

drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2.o: In function `mlxfw_mfa2_xz_dec_run':
:(.text.mlxfw_mfa2_xz_dec_run+0x8): undefined reference to `xz_dec_run'
drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2.o: In function `mlxfw_mfa2_file_component_get':
:(.text.mlxfw_mfa2_file_component_get+0x218): undefined reference to `xz_dec_init'
:(.text.mlxfw_mfa2_file_component_get+0x2c0): undefined reference to `xz_dec_end'

This adds a Kconfig 'select' statement for it, which is also what
the other user of that library has.

Fixes: 410ed13c ("Add the mlxfw module for Mellanox firmware flash process")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>