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>