errno.h header is required now that errno is a per-thread variable.
"undefined symbol 'errno'" at runtime with dlopen()'d modules.

--- backend/plustek-usb.c.orig	Wed May 18 14:00:59 2016
+++ backend/plustek-usb.c	Wed May 18 14:01:09 2016
@@ -1,3 +1,5 @@
+#include <errno.h>
+
 /*.............................................................................
  * Project : SANE library for Plustek flatbed scanners.
  *.............................................................................