create-diff-object: fix for unincluded sec->secsym

When running kpatch-build with -d, I was getting a seg fault.  It was
faulting in kpatch_dump_kelf() when trying to print sec->secsym->name
for the .smp_locks section.  It turns out that the section was included
but its section symbol wasn't included, so sec->secsym pointed to freed
memory.
Merge pull request #301 from jpoimboe/debug-seg-fault

create-diff-object: fix for unincluded sec->secsym