efi_loader: add definition for efi_main()

U-Boot provides multiple EFI applications. The entry point is called
efi_main(). Provide a definition for this function. This avoids
build warnings like

    lib/efi_loader/initrddump.c:468:21: warning:
    no previous prototype for ‘efi_main’ [-Wmissing-prototypes]
      468 | efi_status_t EFIAPI efi_main(efi_handle_t image_handle,
          |                     ^~~~~~~~

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>