updated can.h to have arduino specific can_frame 3 bytes smaller than can_frame structure used by other compilers

[arduinoenigma]

can_frame was changed in Pull Request (#122) to accommodate other compilers by adding alignas(8) to __u8 data[CAN_MAX_DLEN];

When compiling in Arduino, this adds 3 extra bytes of ram per each can_frame struct used.

This pull request detects if the arduino headers are used and then defines an arduino specific version

#if defined(Arduino_h)
struct can_frame {
               canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
               __u8    can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */
			   __u8    data[CAN_MAX_DLEN];
};
#else
struct can_frame {
               canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
               __u8    can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */
    alignas(8) __u8    data[CAN_MAX_DLEN];
};
#endif

With alignas(8):

Serial.println(sizeof(can_frame));
16

Sketch uses 22854 bytes (74%) of program storage space. Maximum is 30720 bytes.
Global variables use 1800 bytes (87%) of dynamic memory, leaving 248 bytes for local variables. Maximum is 2048 bytes.

Without alignas(8):

Serial.println(sizeof(can_frame));
13

Sketch uses 22864 bytes (74%) of program storage space. Maximum is 30720 bytes.
Global variables use 1688 bytes (82%) of dynamic memory, leaving 360 bytes for local variables. Maximum is 2048 bytes.