cleanup header inclusions in API

samples/mrwthumb.cpp

@@ -2,11 +2,13 @@
 // mrwthumb.cpp
 // Sample program to extract a Minolta thumbnail from the makernote
 
-#include <cassert>
 #include "error.hpp"
 #include "exif.hpp"
 #include "image.hpp"
 
+#include <cassert>
+#include <iostream>
+
 int main(int argc, char* const argv[])
 {
     try {

samples/xmpdump.cpp

@@ -5,6 +5,8 @@
 #include <cassert>
 #include <iostream>
 #include <string>
+
+#include "error.hpp"
 #include "image.hpp"
 
 int main(int argc, char* const argv[])