gdb: move lm_info to solib in dsbt_current_sos

Commit 8971d2788e79 ("gdb: link so_list using intrusive_list")
mistakenly removed the line that moves the lm_info unique pointer to
sop->lm_info, probably due to a bad conflict resolution.  Restore that
line.

Unfortunately, this code is only used for TI C66, which is not widely
tested (if used at all).

Change-Id: I9f64eb4430c324bc93ddb4bd00d820dee34adfbb
Approved-By: Tom Tromey <tom@tromey.com>