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/sim-signal.c

@@ -21,6 +21,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "defs.h"
 
 #include <signal.h>
+
 #include "sim-main.h"
 #include "sim-signal.h"