Replace cast from int to void* via 'long' with 'intptr_t'

* src/regex/regcomp.c (parse_dup_op, mark_opt_subexp): Replace
  cast from int to void* via 'long' with 'intptr_t' to fix build
  mingw64 warning cast to pointer from integer of different size
  [-Wint-to-pointer-cast]

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>