cpu/stellaris: fix include guards.

cpu/stellaris_common/include/stellaris_periph/i2c.h

cpu/stellaris_common/include/stellaris_periph/interrupt.h

cpu/stellaris_common/include/stellaris_periph/pin_map.h

cpu/stellaris_common/include/stellaris_periph/pwm.h

cpu/stellaris_common/include/stellaris_periph/rom.h

cpu/stellaris_common/include/stellaris_periph/rom_map.h

cpu/stellaris_common/include/stellaris_periph/ssi.h

cpu/stellaris_common/include/stellaris_periph/sysctl.h

cpu/stellaris_common/include/stellaris_periph/sysexc.h

cpu/stellaris_common/include/stellaris_periph/systick.h

cpu/stellaris_common/include/stellaris_periph/timer.h

cpu/stellaris_common/include/stellaris_periph/uart.h

cpu/stellaris_common/include/stellaris_periph/watchdog.h