Made the copy-tests compile on osx.

lib/c/test/test_labcomm_copy.c

@@ -2,6 +2,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <unistd.h>
 #include <fcntl.h>
 #include <assert.h>
 #include <err.h>