Post-merge compilation fixes.

examples/main.c

examples/main_fof.c

src/black_holes/Default/black_holes_io.h

src/black_holes/EAGLE/black_holes_iact.h

src/black_holes/EAGLE/black_holes_part.h

src/black_holes/EAGLE/black_holes_struct.h

src/common_io.c

src/fof.c

src/gravity/MultiSoftening/gravity_io.h

src/gravity/MultiSoftening/gravity_part.h

src/gravity_properties.c

src/gravity_properties.h

src/parallel_io.c

src/serial_io.c

src/single_io.c