pic16bit: Add definition of SEEK_SET to unistd.h.

Author: dpgeorge

pic16bit/unistd.h

@@ -3,6 +3,7 @@
 
 // XC16 compiler doesn't seem to have unistd.h file
 
+#define SEEK_SET 0
 #define SEEK_CUR 1
 
 typedef int ssize_t;