Fix #1521 - add mg_tou64()

src/str.h

@@ -41,4 +41,5 @@ size_t mg_asprintf(char **, size_t, const char *fmt, ...) PRINTF_LIKE(3, 4);
 size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap);
 int mg_check_ip_acl(struct mg_str acl, uint32_t remote_ip);
 int64_t mg_to64(struct mg_str str);
+uint64_t mg_tou64(struct mg_str str);
 size_t mg_lld(char *buf, int64_t val, bool is_signed, bool is_hex);