temporarily disable FORTIFY in header-abi-dumper

Bionic's FORTIFY implementation expects clang >= r285906, but
header-abi-dumper uses external/clang, which is a clone of clang at
r275480. This causes header-abi-dumper to emit errors in cases where
clang-3688880 doesn't (e.g.

unsigned char c[2];
// error: no matching function, since clang r275480 doesn't like
// unsigned char * -> char * casts
snprintf(c, sizeof(c), "a");).

Bug: 36076947
Test: Ran header-abi-dumper on external/e2fsprogs/lib/blkid/probe.c
(which previously gave us errors). It now only emits a few warnings.

Change-Id: I9460853b7f94aeb154a4d921a852954d82394055