[clangd] Fix -Wunused-variable of 'FIDIt' (NFC)

/data/llvm-project/clang-tools-extra/clangd/index/SymbolCollector.cpp:878:16: error: unused variable 'FIDIt' [-Werror,-Wunused-variable]
    const auto FIDIt = IncludeFiles.find(SID);
               ^
1 error generated.