examples: fix -Wformat warnings

When size_t is not a typedef for unsigned long (as usually the case on
Windows), GCC emits -Wformat warnings when using lu and lx format
specifiers with size_t. Silence them with explicit casts to
unsigned long.

Closes https://github.com/curl/curl/pull/2721

docs/examples/getinmemory.c

@@ -100,7 +100,7 @@ int main(void)
      * Do something nice with it!
      */
 
-    printf("%lu bytes retrieved\n", chunk.size);
+    printf("%lu bytes retrieved\n", (unsigned long)chunk.size);
   }
 
   /* cleanup curl stuff */