cpu: added `extern "C"` to headers

cpu/arm7_common/include/VIC.h

cpu/arm7_common/include/arm7_common.h

cpu/arm7_common/include/hwtimer_cpu.h

cpu/arm7_common/include/iap.h

cpu/atmega2560/include/cpu-conf.h

cpu/atmega2560/include/hwtimer_cpu.h

cpu/atmega_common/avr-libc-extra/time.h

cpu/atmega_common/include/cpu.h

cpu/atmega_common/include/sys/time.h

cpu/atmega_common/include/sys/types.h

cpu/cc430/include/cc430-adc.h

cpu/cc430/include/cc430-rtc.h

cpu/cortex-m0_common/include/core_cm0.h

cpu/cortex-m0_common/include/core_cm0plus.h

cpu/cortex-m0_common/include/core_cmFunc.h

cpu/cortex-m0_common/include/core_cmInstr.h

cpu/cortex-m0_common/include/cpu.h

cpu/cortex-m3_common/include/core_cm3.h

cpu/cortex-m3_common/include/core_cmFunc.h

cpu/cortex-m3_common/include/core_cmInstr.h