scripts/tags.sh: fix direct execution of scripts/tags.sh

I thought this script was run via "make tags" etc. but some people
run it directly.

Prior to commit a9a49c2a ("kbuild: use $(srctree) instead of
KBUILD_SRC to check out-of-tree build"), in such a usecase, "tree"
was set empty since KBUILD_SRC is undefined. Now, "tree" is set to
"${srctree}/", which is evaluated to "/".

Fix it by taking into account the case where "srctree" is unset.

Link: https://lkml.org/lkml/2019/4/19/501


Fixes: a9a49c2a ("kbuild: use $(srctree) instead of KBUILD_SRC to check out-of-tree build")
Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>