fix bug in Thread.cpp/Thread.h/components.c files

src/components.c

@@ -153,7 +153,6 @@ int __low_level_init(void)
     return 0;
 }
 #elif defined(__GNUC__)
-extern int main(void);
 /* Add -eentry to arm-none-eabi-gcc argument */
 int entry(void)
 {
@@ -204,7 +203,7 @@ void rt_application_init(void)
     result = rt_thread_init(tid, "main", main_thread_entry, RT_NULL,
                             main_stack, sizeof(main_stack), RT_MAIN_THREAD_PRIORITY, 20);
     RT_ASSERT(result == RT_EOK);
-	
+
     /* if not define RT_USING_HEAP, using to eliminate the warning */
     (void)result;
 #endif