fix(gdt): include gdt.h in k.c to remove compile warning

Signed-off-by: Julien CLEMENT <julien.clement@epita.fr>

k/k.c

@@ -26,6 +26,7 @@
 #include "multiboot.h"
 #include "stdio.h"
 #include "serial.h"
+#include "gdt.h"
 
 static void k_init(void)
 {