package/libssh2: switch site to https://www.libssh2.org/download

package/libssh2: switch site to https://libssh2.org/download

As spotted by Danomi during review of "libssh2: security bump to version
1.9.0" (https://patchwork.ozlabs.org/patch/1148776), it seems that
the tarball from github and libssh2.org/download are not the same. One
of the difference is that LIBSSH2_VERSION in include/libssh2.h is set to
"1.9.0_DEV" in github tarball whereas it is set to "1.9.0" in
libssh2.org/download.

So switch site to https://www.libssh2.org/download to get "official"
release

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>