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/sh/interp.c

@@ -25,9 +25,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <signal.h>
-#ifdef HAVE_UNISTD_H
 #include <unistd.h>
-#endif
 #ifdef HAVE_MMAP
 #include <sys/mman.h>
 # ifndef MAP_FAILED