selftests/bpf: Define SYS_PREFIX for riscv

SYS_PREFIX was missing for a RISC-V, which made a couple of kprobe
tests fail.

Add missing SYS_PREFIX for RISC-V.

Fixes: 08d0ce30

 ("riscv: Implement syscall wrappers")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/bpf/20231004110905.49024-3-bjorn@kernel.org