Unused variable error when compiling with extra errors

[ozhang05]

include/sol/stack_check_unqualified.hpp:585:75: Warning Unused Variable 'indextype'

Very minor error, but when compiling with extra error flags (-Wall -Wextra -Werror) reveals an unused variable error at line 585. Seemed to fix it for me by deleting the variable (unsure if this breaks other things), or just adding a (void) indextype.