cmake: search LIBSSH_INCLUDE, LIBSSH_LIBRARY paths for libssh files
cmake: search LIBSSH_INCLUDE, LIBSSH_LIBRARY paths for libssh files

The README file mentions that the path to the libssh library can
be specified by defining the LIBSSH_INCLUDE and LIBSSH_LIBRARY variables
but CMAKE wasn't searching in those paths for libssh.

Add those variables to the directory search list for libssh include and
lib files.

rtrlib/rtrlib,rtrlib/rtrlib,rtrlib/rtrlib