examples: Fix include in simple_recon2d.

Add missing include of `iostream` which led to a compilation error.
Place the include according to the used systematic in the other example
code (after the local includes, surrounded by single empty lines).

examples/simple_recon2d.cpp

 /// [simplerecon header begin]
 #include "elsa.h"
 
+#include <iostream>
+
 using namespace elsa;
 /// [simplerecon header end]