Ignore -Wformat warning

Clang 6.0.1 update introduced a number of new warnings, failing
werror build. Ignore the -Wformat warning for external/avb.

Test: m checkbuild
Change-Id: Ic64e64123a707b4cb9d10af107a237b58d7187fd
diff --git a/Android.bp b/Android.bp
index 646a884..4675965 100644
--- a/Android.bp
+++ b/Android.bp
@@ -27,6 +27,7 @@
         "-Wmissing-prototypes",
         "-Wno-psabi",
         "-Wno-unused-parameter",
+        "-Wno-format",
         "-ffunction-sections",
         "-fstack-protector-strong",
         "-g",