Add virtual destruction file to make gcc 4.7 can compile it

BUG=None
TEST=compile passed

Change-Id: I4b4fb763aa820e7096a78087ddd6e87e900a347c
Reviewed-on: https://gerrit.chromium.org/gerrit/21667
Reviewed-by: Will Drewry <wad@chromium.org>
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
diff --git a/file_hasher.h b/file_hasher.h
index e26ef96..48e075e 100644
--- a/file_hasher.h
+++ b/file_hasher.h
@@ -41,6 +41,7 @@
   }
   virtual const char *salt(void) { return salt_; }
 
+  virtual ~FileHasher() {};
   static int WriteCallback(void *file,
                            sector_t start,
                            u8 *dst,