libssh: fix download location

libssh: fix download location

The current download location fails, and Buildroot falls back to
sources.b.o:

--2018-08-20 23:41:39--  https://red.libssh.org/attachments/download/218/libssh-0.7.5.tar.xz
Resolving red.libssh.org (red.libssh.org)... 78.46.80.163
Connecting to red.libssh.org (red.libssh.org)|78.46.80.163|:443... connected.
The certificate's owner does not match hostname ‘red.libssh.org’
--2018-08-20 23:41:39--  http://sources.buildroot.net/libssh/libssh-0.7.5.tar.xz
Resolving sources.buildroot.net (sources.buildroot.net)... 104.25.211.19, 104.25.210.19, 2400:cb00:2048:1::6819:d313, ...
Connecting to sources.buildroot.net (sources.buildroot.net)|104.25.211.19|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 351632 (343K) [application/x-xz]

This commit fixes the download location:

--2018-08-20 23:43:04--  https://www.libssh.org/files/0.7/libssh-0.7.5.tar.xz
Resolving www.libssh.org (www.libssh.org)... 87.98.168.187, 2001:41d0:2:f80c::4
Connecting to www.libssh.org (www.libssh.org)|87.98.168.187|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 351632 (343K) [application/x-tar]

This patch is extracted from a contribution from Bernd Kuhls who was
also bumping the package at the same time
(http://patchwork.ozlabs.org/patch/959192/).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>