unistd does not always include getopt.h (by @velichkov)

skeletons/converter-example.c

@@ -14,7 +14,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <stdlib.h>    /* for atoi(3) */
-#include <unistd.h>    /* for getopt(3) */
+#include <getopt.h>    /* for getopt(3) */
 #include <string.h>    /* for strerror(3) */
 #include <sysexits.h>    /* for EX_* exit codes */
 #include <errno.h>    /* for errno */