In C++ pendantic compilation source, use actual C++ code

In C++ pendantic compilation source, use actual C++ code instead
of relying on C code working when compiled as C++ code.

tests/test-pedantic-compilation.cxx

@@ -15,12 +15,12 @@
 #include <gphoto2/gphoto2-port-result.h>
 #include <gphoto2/gphoto2-port-version.h>
 
-#include <stdio.h>
+#include <iostream>
 
 unsigned long cxx_version = __cplusplus;
 
 int main()
 {
-  printf("cxx_version = %lu\n", cxx_version);
+  std::cout << "cxx_version = " << cxx_version << std::endl;
   return 0;
 }