can: uapi: can.h: mark union inside struct can_frame packed

In commit ea780056 ("can: add optional DLC element to Classical
CAN frame structure") the struct can_frame::can_dlc was put into an
anonymous union with another u8 variable.

For various reasons some members in struct can_frame and canfd_frame
including the first 8 byes of data are expected to have the same
memory layout. This is enforced by a BUILD_BUG_ON check in af_can.c.

Since the above mentioned commit this check fails on ARM kernels
compiled with the ARM OABI (which means CONFIG_AEABI not set). In this
case -mabi=apcs-gnu is passed to the compiler, which leads to a
structure size boundary of 32, instead of 8 compared to CONFIG_AEABI
enabled. This means the the union in struct can_frame takes 4 bytes
instead of the expected 1.

Rong Chen illustrates the problem with pahole in the ARM OABI case:

| struct can_frame {
|          canid_t                    can_id;               /* 0     4 */
|          union {
|                  __u8      ...