kselftest/arm64: Add parentheses around sizeof for clarity

Added parentheses around sizeof to make the expression clearer
and improve readability. This change has no functional impact.

```
[command]
	./scripts/checkpatch.pl tools/testing/selftests/arm64/fp/sve-ptrace.c

[output]
	WARNING: sizeof *sve should be sizeof(*sve)
```

Signed-off-by: Vivek Yadav <vivekyadav1207731111@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>