tools: selftests: riscv: Add test count for vstate_prctl

Add the test count to drop the warning message.
"Planned tests != run tests (0 != 1)"

Fixes: 7cf6198 ("selftests: Test RISC-V Vector prctl interface")
Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andy Chiu <AndybnAC@gmail.com>
Link: https://lore.kernel.org/r/20241220091730.28006-3-yongxuan.wang@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: yong-xuan

tools/testing/selftests/riscv/vector/vstate_prctl.c

@@ -76,6 +76,8 @@ int main(void)
 	long flag, expected;
 	long rc;
 
+	ksft_set_plan(1);
+
 	pair.key = RISCV_HWPROBE_KEY_IMA_EXT_0;
 	rc = riscv_hwprobe(&pair, 1, 0, NULL, 0);
 	if (rc < 0) {