fix const-correctness of argument to stime

it's unclear what the historical signature for this function was, but
semantically, the argument should be a pointer to const, and this is
what glibc uses. correct programs should not be using this function
anyway, so it's unlikely to matter.

include/time.h

@@ -125,7 +125,7 @@ struct tm *getdate (const char *);
 
 
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
-int stime(time_t *);
+int stime(const time_t *);
 time_t timegm(struct tm *);
 #endif