Fix "gmtdigitize.c:77:2: error: unknown type name 'gid_t'"
build failure; patch from upstream version 4.5.11

https://bugs.gentoo.org/show_bug.cgi?id=543624

--- a/src/misc/gmtdigitize.c
+++ b/src/misc/gmtdigitize.c
@@ -37,6 +37,11 @@
  *
  */
  
+#define _XOPEN_SOURCE
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
 #include "gmt.h"
 
 #ifndef WIN32