unifdef: Fix collision check when adding symbols.

findsym() is intended for use while parsing input, so it should not be
called from addsym2() or indirectsym(), which are called before any
input is processed.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D48733

(cherry picked from commit c63af363c2458aebc30c01cd0b93b4b902580019)