Fixed linkage bug

With the new version of GCC(10.1.0) there was a problem about a
variable definition.

kernel/drivers/timer.c

@@ -6,7 +6,6 @@
 
 // Start tick at zero
 uint32_t tick = 0;
-
 static void timer_callback(registers_t regs) {
     tick++;
     /* uint8_t buf[8];
@@ -37,4 +36,4 @@ void init_timer(uint32_t frequency) {
     // Send the frequency divisor
     outb(0x40, low);
     outb(0x40, high);
-}
+}

kernel/drivers/timer.h

@@ -31,11 +31,11 @@
  */
 
 void init_timer(uint32_t frequency);
-uint32_t tick;
+extern uint32_t tick;
 /* Since regs parameter(from timer_callback) will be unused
  * GCC(with -Werror flag) will throw an error, so we can avoid this
  * using the following macro
  */
 #define UNUSED_PAR(x) (void)(x)
 
-#endif
+#endif