tuklib_exit: Add missing header.

strerror() needs <string.h> which happened to be included via
tuklib_common.h -> tuklib_config.h -> sysdefs.h if HAVE_CONFIG_H
was defined. This wasn't tested without config.h before so it
had worked fine.

src/common/tuklib_exit.c

@@ -14,6 +14,7 @@
 
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 
 #include "tuklib_gettext.h"
 #include "tuklib_progname.h"