Change: Make OPENVAS_ENCAPS_TLSv13 visible for nasl scripts (#914)

Support for TLSv13 was added some time ago, but the integer variable was still not visible for nasl script
For testing:
- Create a simple nasl script with "display(ENCAPS_TLSv13);"
- run it with openvas-nasl. 

It should print 8. Without the patch, it doesn't print anything.

nasl/nasl_init.c

@@ -420,6 +420,7 @@ static struct
   {"ENCAPS_TLSv1", OPENVAS_ENCAPS_TLSv1},
   {"ENCAPS_TLSv11", OPENVAS_ENCAPS_TLSv11},
   {"ENCAPS_TLSv12", OPENVAS_ENCAPS_TLSv12},
+  {"ENCAPS_TLSv13", OPENVAS_ENCAPS_TLSv13},
   {"ENCAPS_TLScustom", OPENVAS_ENCAPS_TLScustom},
   {"ENCAPS_MAX", OPENVAS_ENCAPS_MAX},
   {"TH_FIN", TH_FIN},