efi: use stub for cache_efi_options_variable if !ENABLE_EFI

../src/core/main.c: In function 'main':
../src/core/main.c:2637:32: error: implicit declaration of function 'cache_efi_options_variable'; did you mean 'systemd_efi_options_variable'? [-Werror=implicit-function-declaration]
                         (void) cache_efi_options_variable();
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~
                                systemd_efi_options_variable