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 <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

tools/testing/selftests/arm64/fp/sve-ptrace.c

···
 {
 	struct user_sve_header *sve;
 	void *p;
-	size_t sz = sizeof *sve;
+	size_t sz = sizeof(*sve);
 	struct iovec iov;
 	int ret;