Fixed linkage bug

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

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