fix stdup declaration

- we need to include sane/config.h first  to use strdup

ChangeLog

@@ -1,3 +1,6 @@
+2015-09-16 Stéphane Voltz <stef.dev@free.fr>
+	* backend/pieusb.c: include sane/config.h first
+
 2015-09-14 Stéphane Voltz <stef.dev@free.fr>
 	* backend/bh.c: replace mktemp by mkstemp #300134
 

backend/pieusb.c

@@ -58,6 +58,7 @@
  *
  * --------------------------------------------------------------------------*/
 
+#include "../include/sane/config.h"
 /* Standard includes for various utiliy functions */
 #include <stdio.h> /* for FILE */
 #include <string.h> /* for strlen */