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/rx/main.c

@@ -24,9 +24,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
-#ifdef HAVE_UNISTD_H
 #include <unistd.h>
-#endif
 #include <assert.h>
 #include <setjmp.h>
 #include <signal.h>