Explicitly note ignored return result

Otherwise Clang will emit -Wunused-but-set-variable warning.

Test: build
Change-Id: I889e6262073db876b9d2da477c3cb33b8f472817
diff --git a/libartbase/base/unix_file/fd_file.cc b/libartbase/base/unix_file/fd_file.cc
index 2102fec..a955b7d 100644
--- a/libartbase/base/unix_file/fd_file.cc
+++ b/libartbase/base/unix_file/fd_file.cc
@@ -557,6 +557,7 @@
   result = Flush();
   result = Close();
   // Ignore the errors.
+  (void) result;
 
   return ret_result;
 }