Updated idt.c `18-interrupts`

Fix to get rid any possible multiple definition errors in `18-interrupts`

18-interrupts/cpu/idt.c

@@ -1,6 +1,9 @@
 #include "idt.h"
 #include "../kernel/util.h"
 
+idt_gate_t idt[IDT_ENTRIES];
+idt_register_t idt_reg;
+
 void set_idt_gate(int n, u32 handler) {
     idt[n].low_offset = low_16(handler);
     idt[n].sel = KERNEL_CS;