Corrected mistake in serial i/o and added missing headed inclusion.

src/hydro/Default/hydro_part.h

src/hydro/Gadget2/hydro_io.h

src/hydro/Gizmo/hydro_part.h

src/hydro/Minimal/hydro_part.h

src/hydro/PressureEntropy/hydro_part.h

src/hydro/Shadowswift/hydro_part.h

src/parallel_io.c

src/serial_io.c

src/single_io.c