i40e: Replace one-element array with flex-array member in struct i40e_package_header

One-element and zero-length arrays are deprecated. So, replace
one-element array in struct i40e_package_header with flexible-array
member.

The `+ sizeof(u32)` adjustments ensure that there are no differences
in binary output.

Link: https://github.com/KSPP/linux/issues/335


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>