Include <ctype.h> for isxdigit(3).

diff -u -r ../poppler-0.59.0/poppler/Form.cc ./poppler/Form.cc
--- ../poppler-0.59.0/poppler/Form.cc	2017-08-16 00:16:09.000000000 +0200
+++ ./poppler/Form.cc	2017-09-11 20:06:45.434163003 +0200
@@ -29,6 +29,7 @@
 
 #include <set>
 #include <limits>
+#include <ctype.h>
 #include <stddef.h>
 #include <string.h>
 #include "goo/gmem.h"