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

Index: backend/p5_device.c
--- backend/p5_device.c.orig
+++ backend/p5_device.c
@@ -1,3 +1,5 @@
+#include <errno.h>
+
 /**
  * Description of the Primax PagePartner model
  */