Clang: Fix compiler warning

"warning: local variable 'filePaths' will be copied despite being
returned by name [-Wreturn-std-move]"
"note: call 'std::move' explicitly to avoid copying"

Change-Id: Ib34fcae4401b65a6d5c4bb2fb79bbf12c1e84f35
Reviewed-by: hjk <hjk@qt.io>

src/tools/clangbackend/source/clangfilesystemwatcher.cpp

@@ -60,7 +60,7 @@ QStringList filterExistingFiles(QStringList &&filePaths)
 
     filePaths.erase(startOfNonExistingFilePaths, filePaths.end());
 
-    return filePaths;
+    return std::move(filePaths);
 }
 }