Re-#include <time.h> in checkpointer.c.

Commit 7dbfea3c45 thought it could get
away with removing this, but Thomas Munro reports, on behalf of the
buildfarm, that it's still needed at least on Windows to avoid
compiler warnings.

src/backend/postmaster/checkpointer.c

@@ -37,6 +37,7 @@
 #include "postgres.h"
 
 #include <sys/time.h>
+#include <time.h>
 
 #include "access/xlog.h"
 #include "access/xlog_internal.h"