sim: formally assume unistd.h always exists (via gnulib)

We have many uses of unistd.h that are unprotected by HAVE_UNISTD_H,
so this is more formalizing the reality that we require this header.
Since we switched to gnulib, it guarantees that a unistd.h exists
for us to include, so we're doubly OK.

sim/common/callback.c

@@ -34,9 +34,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <time.h>
-#ifdef HAVE_UNISTD_H
 #include <unistd.h>
-#endif
 #include <sys/stat.h>
 #include <sys/types.h>