Fix compilation warnings in the LPEG submodule

[rdw-software]

May only happen on Windows? Or perhaps I missed it when working inside WSL...

[5/52] Compiling deps/lpeg/lptree.c ...
deps/lpeg/lptree.c: In function 'addtoktable':
deps/lpeg/lptree.c:166:11: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
  166 |     if (n >= USHRT_MAX)
      |           ^~
deps/lpeg/lptree.c: In function 'concattable':
deps/lpeg/lptree.c:198:15: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
  198 |   if (n1 + n2 > USHRT_MAX)
      |               ^