modpost: explain why we can't use strsep

mainline inclusion
from mainline-5.8
commit 6020db50
category: bugfix
bugzilla: 46847
CVE: NA

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Mention why we open-code strsep, so it is clear that it is intentional.

Fixes: 736bb118 ("modpost: remove use of non-standard strsep() in HOSTCC code")
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Jian Cheng <cj.chengjian@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>