Use stdint.h for int64_t / uint64_t in API.

Now that we can use stdint.h, we can use it to portably define
64 bit integer types for use in the API.

src/api/z3.h

@@ -23,6 +23,7 @@ Notes:
 
 #include <stdio.h>
 #include <stdbool.h>
+#include <stdint.h>
 #include "z3_macros.h"
 #include "z3_api.h"
 #include "z3_ast_containers.h"