[OpenMP] Fix compile error caused by #74397 (#75419)

This patch fixed the following compile error caused by #74397.

```
openmp/libomptarget/src/PluginManager.cpp: In static member function ‘static llvm::Expected<std::unique_ptr<PluginAdaptorTy> >
PluginAdaptorTy::create(const string&)’:
openmp/libomptarget/src/PluginManager.cpp:50:10: error: could not convert ‘PluginAdaptor’ from ‘std::unique_ptr<PluginAdaptorTy
>’ to ‘llvm::Expected<std::unique_ptr<PluginAdaptorTy> >’
   return PluginAdaptor;                                                                                                                                                                    ^~~~~~~~~~~~~
```