modpost: explain why we can't use strsep

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>