converters/pdf2djvu: fix build on GCC architectures

When compiling with GCC, as used on GCC architectures, it errors with:
sys-uuid.cc:96:12: error: 'strlen' was not declared in this scope

assert(strlen(s) == 36U);
       ^~~~~~

sys-uuid.cc:96:12: note: 'strlen' is defined in header '<cstring>'; did you forget to '#include <cstring>'?

PR:		238780
Approved by:	samy.mahmoudi@gmail.com (maintainer timeout), tcberner (mentor)
Differential Revision:	https://reviews.freebsd.org/D22025

converters/pdf2djvu/files/patch-sys-uuid.cc

@@ -0,0 +1,11 @@
+--- sys-uuid.cc.orig	2019-06-23 11:41:41 UTC
++++ sys-uuid.cc
+@@ -17,6 +17,8 @@
+ #include "autoconf.hh"
+ #include "system.hh"
+ 
++#include <cstring>
++
+ #if WIN32
+ 
+ #include <cassert>