imx: hab: Make usage of packed attribute consistent

commit cd2d4600 ("arm: imx: hab: Add IVT header definitions") declares
struct ivt_header as "__attribute__((packed))".

commit ed286bc8 ("imx: hab: Check if CSF is valid before
authenticating image") declares struct hab_hdr with __packed.

This patch makes the __packed convention consistent.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Cc: Breno Lima <breno.lima@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>