boards/native_sim: Deprecate timer_model.h

Deprecate this header which provides API compatibility for the
native_posix timer HW model, with planned removal in v4.4.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

Author: aescolar

boards/native/native_sim/timer_model.h

@@ -7,6 +7,9 @@
 #ifndef BOARDS_POSIX_NATIVE_SIM_TIMER_MODEL_H
 #define BOARDS_POSIX_NATIVE_SIM_TIMER_MODEL_H
 
+#warning "This transitional header is now deprecated and will be removed by v4.4. "\
+	 "Use nsi_timer_model.h instead."
+
 /*
  * To support the native_posix timer driver
  * we provide a header with the same name as in native_posix