sim: include stdint.h when needed

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

sim/common/callback.c

@@ -29,6 +29,7 @@
 #include <limits.h>
 #include <signal.h>
 #include <stdarg.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>