HTIF symbols no longer recognized by RISC-V spike board

Tested commit: f34edbc760b0f689deddd175fc08732ecb46665f

I belive this was introduced in 0ac24d56c5e7d32423ea78ac58a06b444d1df04d when the spike's load_kernel() was moved to riscv_load_kernel() which no longer included htif_symbol_callback().