Fixes#165413. Where a build failure was reported:
```
/b/s/w/ir/x/w/llvm-llvm-project/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp:1182:9: error: unknown type name 'user_sve_header'; did you mean 'sve::user_sve_header'?
1182 | user_sve_header *header =
| ^~~~~~~~~~~~~~~
| sve::user_sve_header
```
To fix this, add sve:: as we do for all other uses of this.
This is LLDB's copy of a structure that Linux also defines. I think the
build worked on some machines because that version ended up being
included, but with a more isolated build, it may not.
We have our own definition of it so we can be sure what we're using in
case Linux extends it later.
5f6835daf4