sim: include stdarg.h when used

Avoid implicit include deps with this to help untangle sim headers
so we can get rid of arch-specific sim-main.h.

sim/common/sim-engine.c

@@ -20,6 +20,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 /* This must come before any other includes.  */
 #include "defs.h"
 
+#include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>