Fix signedness of type of pointer passed to snprintf

snprintf takes a char* but the var was a uint8_t, which is unsigned char*.

Suggested-by: gcc -Wpointer-sign
Suggested-by: https://github.com/csutils/csmock
See-also: #6

src/libpst.c

@@ -4477,11 +4477,11 @@ void pst_rfc2231(pst_string *str) {
     strcpy(buffer, "utf-8''");
     x = (int8_t *)str->str;
     const uint8_t *y = (uint8_t *)str->str;
-    uint8_t *z = (uint8_t *)buffer;
+    char *z = buffer;
     z += strlen(buffer);    // skip the utf8 prefix
     while (*y) {
         if (*x <= 32) {
-            *(z++) = (uint8_t)'%';
+            *(z++) = '%';
             snprintf(z, 3, "%2" PRIx8, *y);
             z += 2;
         }