selftests/bpf: Define SYS_NANOSLEEP_KPROBE_NAME for riscv

Add missing sys_nanosleep name for RISC-V, which is used by some tests
(e.g. attach_probe).

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/20231004110905.49024-4-bjorn@kernel.org