gnu: libssh: Restore support for cross-compilation.

Partly fixes <https://issues.guix.gnu.org/78029>.

Fixes a regression introduced in
b12d44dd5e where cross-compiling would no
longer work due to CMocka not being found.

* gnu/packages/ssh.scm (libssh)[arguments]: Pass “-DUNIT_TESTING=ON”
only when (%current-target-system) is #f.

Reported-by: Yelninei <yelninei@tutamail.com>
Change-Id: I9c7af784eabb2ae1d6022b134e0252a1e2952bd4

gnu/packages/ssh.scm

@@ -161,7 +161,9 @@ file names.
      (list
       #:configure-flags
       #~(list "-DWITH_GCRYPT=ON"
-              "-DUNIT_TESTING=ON"
+              #$@(if (%current-target-system)
+                     #~()
+                     #~("-DUNIT_TESTING=ON"))
               #$@(if (and (%current-target-system)
                           (not (target-64bit?)))
                      #~((string-append