net-libs/libssh2: Restore slot dependency after OpenSSL uprev.

This reverts commit 5cf9ab8ddb7f760058f1d06f1f4837953d94bf7f.

BUG=chromium:1096089
TEST=CQ

Change-Id: I54c122b1175da67445fafa84534d0b0c442bbcbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2542974
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
1 file changed