fix missing string.h in strftime.c (needed by new strftime code)

this bug was masked by local experimental CFLAGS in my config.mak.

src/time/strftime.c

 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <langinfo.h>
 #include <locale.h>
 #include <time.h>