output: include stdlib.h to avoid compilation error

The stdlib.h has EXIT_FAILURE and free(3).

Reported-by: Sebastián Cramatte <scramatte@nixus.es>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

src/mustach-dhcpd-pools.c

@@ -42,6 +42,7 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <error.h>
+#include <stdlib.h>
 #include <sys/stat.h>
 #include <unistd.h>