Make code const correct

include/fmpi_core.h

include/fmpi_task.h

include/internal/fmpi_mpi.h

src/fmpi_core.c

src/fmpi_ctx.c

src/fmpi_futhark.c

src/fmpi_mpi.c

src/fmpi_stencil.c

src/fmpi_task.c