iostream: Move include of asterisk.h

The asterisk.h header file needs to be included first or else
some things go awry, such as:

implicit declaration of function 'vasprintf'

Change-Id: I981dc2a77a1ba791888e4f1726644d4656c0407c

main/iostream.c

@@ -16,10 +16,11 @@
  * at the top of the source tree.
  */
 
+#include "asterisk.h"
+
 #include <fcntl.h>
 #include <stdarg.h>
 
-#include "asterisk.h"
 #include "asterisk/utils.h"
 #include "asterisk/astobj2.h"
 #include "asterisk/iostream.h"