Fixed gcc-4.7 building problem for update_engine (again).

TEST=Built using gcc-4.7 for lumpy.
BUG=None

Change-Id: Ic5dc527044d75549d7a125c64990bbee4d2dfa21
Reviewed-on: https://gerrit.chromium.org/gerrit/26138
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Ready: Han Shen <shenhan@chromium.org>
Tested-by: Han Shen <shenhan@chromium.org>
diff --git a/SConstruct b/SConstruct
index fd3fa18..43d4392 100644
--- a/SConstruct
+++ b/SConstruct
@@ -184,6 +184,7 @@
                              -Wignored-qualifiers
                              -Wignored-qualifiers
                              -Wmissing-field-initializers
+                             -Wno-format
                              -Wsign-compare
                              -Wtype-limits
                              -Wtype-limits