riscv/boot: Add a missing header include

riscv/boot: Add a missing header include

As the functions declared in this header use the symbol_fn_t
typedef itself declared in "hw/loader.h", we need to include
it here to make the header file self-contained.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>