net/remmina: Fix build with security/libssh 0.8 branch

libssh 0.8.x no longer installs libssh_threads.so [1], [2], [3].
The upstream remmina code has been updated to accomodate this [4].
Therefore the remmina port Makefiles simply need to be updated to depend
on libssh.so instead of libssh_threads.so.

[1] https://git.libssh.org/projects/libssh.git/commit/?id=83b43443e51b5db06184750fb874e1e8d7ece95a
[2] https://git.libssh.org/projects/libssh.git/commit/?id=8425dce7b27d9e6868735b7ca6dc4e50094af9fa
[3] https://git.libssh.org/projects/libssh.git/commit/?id=d0f3cdfa10436d2108e0b75aad53ce976db3e546
[4] https://github.com/FreeRDP/Remmina/commit/8d654eca784d5e1b2c6e853b7beb7ad3d8322290#diff-b467acb9761e344e80ab07b9e7325b66

PR: 232376
Reported by: Graham Perrin <grahamperrin@gmail.com>