Remove some hacks from libcurl sample

samples/libcurl/callbacks.h

@@ -1,8 +1,5 @@
-#include <stdio.h>
+#include <stddef.h>
 
 // Those types are needed but not defined or used in libcurl headers.
 typedef size_t header_callback(char *buffer,   size_t size,   size_t nitems, void *userdata);
 typedef size_t write_data_callback(void *buffer, size_t size, size_t nmemb, void *userp);
-
-extern void something_using_callback1(header_callback*);
-extern void something_using_callback2(write_data_callback*);