NetBSD warning

[XVilka]

../subprojects/libdemangle/src/microsoft_demangle.c:257:8: warning: array subscript is of type 'char' [-Wchar-subscripts]
                        if (isdigit(*sym)) {
                            ^~~~~~~~~~~~~
/usr/include/sys/ctype_inline.h:51:44: note: expanded from macro 'isdigit'
#define isdigit(c)      ((int)((_ctype_tab_ + 1)[(c)] & _CTYPE_D))
                                                ^~~~
../subprojects/libdemangle/src/microsoft_demangle.c:2005:6: warning: array subscript is of type 'char' [-Wchar-subscripts]
        if (isdigit(*curr_pos)) {
            ^~~~~~~~~~~~~~~~~~
/usr/include/sys/ctype_inline.h:51:44: note: expanded from macro 'isdigit'
#define isdigit(c)      ((int)((_ctype_tab_ + 1)[(c)] & _CTYPE_D))
                                                ^~~~
../subprojects/libdemangle/src/microsoft_demangle.c:2019:13: warning: array subscript is of type 'char' [-Wchar-subscripts]
        } else if (isalpha(*curr_pos)) {
                   ^~~~~~~~~~~~~~~~~~
/usr/include/sys/ctype_inline.h:49:44: note: expanded from macro 'isalpha'
#define isalpha(c)      ((int)((_ctype_tab_ + 1)[(c)] & _CTYPE_A))
                                                ^~~~
3 warnings generated.