explicitly convert string pointer type

src/kernel_main.c

@@ -10,12 +10,12 @@ void kernel_main(void)
     result = check_a20_on();
 
     if (result) {
-        vga_printk("ON", 0x0fU);
+        vga_printk((const int8_t*)"ON", 0x0fU);
     } else {
-        vga_printk("NOT ON", 0x0fU);
+        vga_printk((const int8_t*)"NOT ON", 0x0fU);
     }
 
-    vga_printk("No work to do, halting...", 0x0fU);
+    vga_printk((const int8_t*)"No work to do, halting...", 0x0fU);
 
 _loop:
     goto _loop;