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/arm/armos.c

@@ -31,10 +31,7 @@
 #include <errno.h>
 #include <limits.h>
 #include <string.h>
-
-#ifdef HAVE_UNISTD_H
 #include <unistd.h>		/* For SEEK_SET etc.  */
-#endif
 
 #include "armdefs.h"
 #include "armos.h"