Include unistd.h for getpid() and unlink() in fuzzer

fuzz/fuzz-conv.c

@@ -1,6 +1,7 @@
 #include <stdint.h>
 #include <string.h>
 #include <stdlib.h>
+#include <unistd.h>
 #include <gc.h>
 #include "wc.h"
 #include "wtf.h"