header-checker: Remove _FORTIFY_SOURCE workaround

This commit removes `-U_FORTIFY_SOURCE` workaround for the old clang.
clang-r339409b is new enough to build bionic w/o problems.

Bug: 117764794
Bug: 111579848
Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py
Change-Id: I94b119a0195d47d74816ab3f92af79aaa04454f8