Add a definition for ICONV_10646 in case it's missing

--- camel/camel-iconv.h	2014-03-24 10:07:52.000000000 +0100
+++ camel/camel-iconv.h	2015-06-01 08:41:44.271620227 +0200
@@ -28,6 +28,9 @@
 #include <sys/types.h>
 #include <iconv.h>
 #include <glib.h>
+#ifndef ICONV_10646
+#define ICONV_10646 "iso-10646"
+#endif
 
 G_BEGIN_DECLS