objtool: Add mark_sec_changed()

Ensure elf->changed always gets set when sec->changed gets set.

Link: https://lore.kernel.org/r/9a810a8d2e28af6ba07325362d0eb4703bb09d3a.1685464332.git.jpoimboe@kernel.org


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>