[lld-macho] Fix warning

```
/Users/ksmiley/dev/llvm-project/lld/MachO/Symbols.cpp:43:27: warning: field 'external' will be initialized after field 'weakDefCanBeHidden' [-Wreorder-ctor]
      weakDef(isWeakDef), external(isExternal),
                          ^
1 warning generated.
```

Differential Revision: https://reviews.llvm.org/D113823