lib/timeutils/timeutils: timeutils_mktime may accept negative time values.

And will normalize them.
diff --git a/lib/timeutils/timeutils.h b/lib/timeutils/timeutils.h
index d6e3536..c3f1fc2 100644
--- a/lib/timeutils/timeutils.h
+++ b/lib/timeutils/timeutils.h
@@ -49,7 +49,7 @@
 mp_uint_t timeutils_seconds_since_2000(mp_uint_t year, mp_uint_t month,
     mp_uint_t date, mp_uint_t hour, mp_uint_t minute, mp_uint_t second);
 
-mp_uint_t timeutils_mktime(mp_uint_t year, mp_uint_t month, mp_uint_t mday,
-    mp_uint_t hours, mp_uint_t minutes, mp_uint_t seconds);
+mp_uint_t timeutils_mktime(mp_uint_t year, mp_int_t month, mp_int_t mday,
+    mp_int_t hours, mp_int_t minutes, mp_int_t seconds);
 
 #endif // __MICROPY_INCLUDED_LIB_TIMEUTILS_H__