gcc-plugins: randstruct: Update code comment in relayout_struct()

Update code comment to clarify that the only element whose layout is
not randomized is a proper C99 flexible-array member. This update is
complementary to commit 1ee60356

 ("gcc-plugins: randstruct: Only
warn about true flexible arrays")

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZWJr2MWDjXLHE8ap@work
Fixes: 1ee60356

 ("gcc-plugins: randstruct: Only warn about true flexible arrays")
Signed-off-by: Kees Cook <keescook@chromium.org>