duration.c:66:6: warning: array subscript is of type 'char' on NetBSD 9

I get the following warning on NetBSD 9.3:

duration.c:66:6: warning: array subscript is of type 'char' [-Wchar-subscripts]
        if (toupper(str[0]) != 'P') {
            ^~~~~~~~~~~~~~~
/usr/include/sys/ctype_inline.h:60:46: note: expanded from macro 'toupper'
#define toupper(c)      ((int)((_toupper_tab_ + 1)[(c)]))
                                                  ^~~~

Fix:

diff --git a/lib/isccfg/duration.c b/lib/isccfg/duration.c
index 27dddca1cc..305047e1af 100644
--- a/lib/isccfg/duration.c
+++ b/lib/isccfg/duration.c
@@ -63,7 +63,7 @@ isccfg_duration_fromtext(isc_textregion_t *source,
        duration->unlimited = false;
 
        /* Every duration starts with 'P' */
-       if (toupper(str[0]) != 'P') {
+       if (toupper((unsigned char)str[0]) != 'P') {
                return (ISC_R_BADNUMBER);
        }
        P = str;