xl2tpd: bump to version 1.3.13

xl2tpd: bump to version 1.3.13

Changelog prepared by upstream maintainer.  It's mostly about code cleanup and
doc amendment

  v1.3.13 (December 3, 2018)
  * Specify email address for reporting security vulnerabilities [Samir Hussain]
  * Fix compile warning with USE_KERNEL in xl2tpd.c [Samir Hussain]
  * Applying patch that reduces compile warnings and fixes warnings from gcc and clang. [Gareth Ansell]
  * Fix compiler warnings in network.c [Gareth Ansell]
  * Add a make command for packaging's prep work [Samir Hussain]
  * Add Makefile directive for getting version [Samir Hussain]
  * Add a preproc for Watchguard firewall (Github issue #136) [daniel1111]
  * Convert from ISO-8859 to UTF-8 [Simon Deziel]
  * Update README to provide latest info on xl2tpd + Linux kernel 4.15+ [Samir Hussain]
  * Use dh_auto_build in order to allow cross compiles [Helmut Grohne]

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>