[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;                                                                                                                                                                    ^~~~~~~~~~~~~
```
GitOrigin-RevId: 54b54d63caa8b672cbc97fe291ad1b4e4f5895a6
diff --git a/libomptarget/src/PluginManager.cpp b/libomptarget/src/PluginManager.cpp
index 1778728..34a0d1d 100644
--- a/libomptarget/src/PluginManager.cpp
+++ b/libomptarget/src/PluginManager.cpp
@@ -47,7 +47,7 @@
       new PluginAdaptorTy(Name, std::move(LibraryHandler)));
   if (auto Err = PluginAdaptor->init())
     return Err;
-  return PluginAdaptor;
+  return std::move(PluginAdaptor);
 }
 
 PluginAdaptorTy::PluginAdaptorTy(const std::string &Name,