Follow-up to 4f91bfa3 ("Find libssh2 via pkg-config"): FindPkgConfig sets
<foo>_INCLUDE_DIRS, not <foo>_INCLUDE_DIR like FindLIBSSH2 did.

Additionally, if using only FindPkgConfig to find libssh2, we have to
call LINK_DIRECTORIES() as well to pass the appropriate -L entries,
otherwise the build will only work if libssh2.so is in a directory
searched automatically by the linker.