Subject: [PATCH] tags.sh: Add missing quotes

When $remove_structs is empty a test for empty string will turn
into test -n with no arguments meaning true. Add quotes so an
empty string is tested and so that make cscope works again.

Reported-and-tested-by: Jike Song <albcamus@gmail.com>
Reported-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: Yang Bai <hamo.by@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>