[flang] Fix -Wunused-but-set-variable in Bridge.cpp (NFC)

llvm-project/flang/lib/Lower/Bridge.cpp:3775:14:
error: variable 'nbDeviceResidentObject' set but not used [-Werror,-Wunused-but-set-variable]
    unsigned nbDeviceResidentObject = 0;
             ^
1 error generated.

Author: DamonFool

flang/lib/Lower/Bridge.cpp

@@ -3772,7 +3772,7 @@ class FirConverter : public Fortran::lower::AbstractConverter {
     localSymbols.pushScope();
     auto transferKindAttr = fir::CUDADataTransferKindAttr::get(
         builder.getContext(), fir::CUDADataTransferKind::DeviceHost);
-    unsigned nbDeviceResidentObject = 0;
+    [[maybe_unused]] unsigned nbDeviceResidentObject = 0;
     for (const Fortran::semantics::Symbol &sym :
          Fortran::evaluate::CollectSymbols(assign.rhs)) {
       if (const auto *details =