消除了libc的_start方法的编译警告

user/libs/libc/sysdeps/x86_64/elf/start.c

@@ -1,10 +1,13 @@
 
-extern int main(int, char**);
-#include<libc/stdio.h>
-void _start(int argc, char** argv)
+#include <libc/stdio.h>
+#include <libc/stdlib.h>
+
+extern int main(int, char **);
+
+void _start(int argc, char **argv)
 {
-    printf("before main\n");
+    // printf("before main\n");
     int retval = main(argc, argv);
-    printf("before exit, code=%d\n", retval);
+    // printf("before exit, code=%d\n", retval);
     exit(retval);
 }