fix standalone sample build

Partial revert of [1: e565f3c73]. The include file core.h is not
exported in the mono build.

1: 2025-09-19 e565f3c732284288ccb01715bd66ed70a8e4b8d8
   rtk: db was not loaded

Change-Id: I12b14479c7806deaa105c5745b307f38b1a9be60

Author: hefloryd

src/ports/linux/mono.c

@@ -15,10 +15,14 @@
 #include "up_api.h"
 #include "up_util.h"
 #include "model.h"
-#include "core.h"
 
 #include <stdio.h>
 
+extern void up_core_init (void);
+extern void up_core_set_status (uint32_t status);
+extern void core_set_interface (char * iface, size_t size);
+extern void core_bringup_network (void);
+
 static void main_entry (up_t * up)
 {
    while (true)

src/ports/rt-kernel/mono.c

@@ -15,7 +15,6 @@
 #include "up_api.h"
 #include "up_util.h"
 #include "model.h"
-#include "core.h"
 
 #include "osal.h"
 
@@ -29,6 +28,10 @@
 #define APP_TASK_PRIO       5
 #define APP_TASK_STACK_SIZE 6000
 
+extern void up_core_init (void);
+extern void up_core_set_status (uint32_t status);
+extern void core_bringup_network (void);
+
 static void main_entry (void * arg)
 {
    up_t * up = (up_t *)arg;

src/ports/windows/mono.c

@@ -15,10 +15,14 @@
 #include "up_api.h"
 #include "up_util.h"
 #include "model.h"
-#include "core.h"
 
 #include <stdio.h>
 
+extern void up_core_init (void);
+extern void up_core_set_status (uint32_t status);
+extern void core_set_interface (char * iface, size_t size);
+extern void core_bringup_network (void);
+
 static void main_entry (up_t * up)
 {
    while (true)