boards/native_sim: Deprecate native_posix_compat.h

Deprecate this header which provides API compatibility with
native_posix, with planned removal in v4.4.

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

Author: aescolar

boards/native/native_sim/native_posix_compat.h

@@ -15,6 +15,9 @@
 #ifndef BOARDS_POSIX_NATIVE_SIM_NATIVE_POSIX_COMPAT_H
 #define BOARDS_POSIX_NATIVE_SIM_NATIVE_POSIX_COMPAT_H
 
+#warning "This transitional header is now deprecated and will be removed by v4.4. "\
+	 "Use nsi_hw_scheduler.h instead."
+
 #include <stdint.h>
 #include <zephyr/toolchain.h>
 #include "nsi_hw_scheduler.h"