error on symbol ambiguity in link-vmlinux-syms

Like in add-patches-section, we should continue to
search the vmlinux symbol table to see if there
is a duplicate global symbol.  This normally would
not be possible, however, because create-diff-object
modifies unchanged local functions to be global
as a hack so they can be linked, there is a chance
that these symbols could collide with an existing
global symbol.  We should detect this case and error
out.

Hopefully we can figure out a way to avoid this
situation altogether.  But for now, this is a
protection against improper linking.

Signed-off-by: Seth Jennings <sjenning@redhat.com>