Fix to build with PEGTL 2.8.3 in anticipation of an update.

doc/release/notes/third-party-pegtl.rst

+Third-party dependencies
+------------------------
+
+PEGTL
+~~~~~
+
+SMTK will now build with either PEGTL v2.7.1 or v2.8.3.
+Our continuous integration machines have had their superbuild
+upgraded to v2.8.3 to fix an issue with parse-trees that would
+lead to duplicate terminal nodes. Using an version of PEGTL
+older than v2.8.3 could lead to incorrect units.