add stdint.h

stdint.h is needed, otherwise you receive the following error: `error: unknown type name ‘uint32_t’`

_parts/part3.md

@@ -247,6 +247,14 @@ Now would be a great time to write some tests, for a couple reasons:
 
 We'll address those issues in the next part. For now, here's the complete diff from this part:
 ```diff
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
++#include <stdint.h>
+
+ struct InputBuffer_t {
+
  typedef struct InputBuffer_t InputBuffer;
 
 +enum ExecuteResult_t { EXECUTE_SUCCESS, EXECUTE_TABLE_FULL };
@@ -426,4 +434,4 @@ We'll address those issues in the next part. For now, here's the complete diff f
 + }
  }
  }
-```
+```