efi_loader: efi_var_mem_notify_exit_boot_services

efi_var_mem_notify_exit_boot_services() is invoked when ExitBootServices()
is called by the UEFI payload.

efi_var_mem_notify_exit_boot_services() should not be defined as
__efi_runtime as it is invoking EFI_ENTRY() and EFI_EXIT() which themselves
are not __efi_runtime.

Fixes: f1f990a8 ("efi_loader: memory buffer for variables")
Fixes: e01aed47 ("efi_loader: Enable run-time variable support for tee based variables")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>