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
Approved by: re (cperciva)

(cherry picked from commit c63af363c2458aebc30c01cd0b93b4b902580019)
(cherry picked from commit 931b0990297e2c6307b41bda95c5225cdf48403c)