sim: reorder header includes

We're including system headers after local headers in a bunch of
places, but this leads to conflicts when our local headers happen
to define symbols that show up in the system headers.

Use the more standard order of:
* config.h (via defs.h)
* system headers
* local library headers (e.g. bfd & libiberty)
* sim specific headers

sim/common/nrun.c

@@ -21,24 +21,23 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <stdlib.h>
 /* For strsignal.  */
 #include <string.h>
+#ifdef HAVE_UNISTD_H
+/* For chdir.  */
+#include <unistd.h>
+#endif
+
+#include "bfd.h"
+#include "environ.h"
 
 #include "sim-main.h"
 #include "sim-signal.h"
 #include "sim/callback.h"
 
-#include "bfd.h"
-#include "environ.h"
-
 #ifndef HAVE_STRSIGNAL
 /* While libiberty provides a fallback, it doesn't provide a prototype.  */
 extern const char *strsignal (int);
 #endif
 
-#ifdef HAVE_UNISTD_H
-/* For chdir.  */
-#include <unistd.h>
-#endif
-
 static void usage (void);
 
 extern host_callback default_callback;