Fix Verilator VCD (#157)

csrc/verilator.h

@@ -1,6 +1,28 @@
 #ifndef _ROCKET_VERILATOR_H
 #define _ROCKET_VERILATOR_H
 
+#include "verilated_vcd_c.h"
+#include <stdlib.h>
+#include <stdio.h>
+
 extern bool verbose;
 
+class VerilatedVcdFILE : public VerilatedVcdFile {
+ public:
+  VerilatedVcdFILE(FILE* file) : file(file) {}
+  ~VerilatedVcdFILE() {}
+  bool open(const string& name) override {
+    // file should already be open
+    return file != NULL;
+  }
+  void close() override {
+    // file should be closed elsewhere
+  }
+  ssize_t write(const char* bufp, ssize_t len) override {
+    return fwrite(bufp, 1, len, file);
+  }
+ private:
+  FILE* file;
+};
+
 #endif