agent: Fix string translation for Windows
* agent/agent.h (L_): Define agent_Lunderscore when simple gettext is used.
This fixes a regression introduced by b3286af3 ENABLE_NLS
is not defined if we use simple gettext and not gettext.
- GnuPG-Bug-Id: T3364
- Signed-off-by: Andre Heinecke <aheinecke@intevation.de>