This enables 'pthread' support on arduino.
avr-libc C90 'time.h' does not include 'sys/types.h' as POSIX expects it.
However, the type previously defined conflicts with the one in
'cpu/atmega_common/avr_libc_extra/include/sys/types.h' when both are
included, so include 'sys/types.h'.
Maybe it should alway be included by 'time.h' but this
would need its specific review.
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