Fix compilation error in simid5100.c

Fixes:
simid5100.c:90:35: error: ‘errno’ undeclared (first use in this function)

simulators/simid5100.c

@@ -10,6 +10,7 @@ struct ip_mreq
 };
 #endif
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>