efi_loader: Select EVENT as well

Fixes

WARNING: unmet direct dependencies detected for EVENT_DYNAMIC
  Depends on [n]: EVENT [=n]
  Selected by [y]:
  - EFI_LOADER [=y] && OF_LIBFDT [=y] && ...

and the succeeding build breakage.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>