selftests/functional/test_output.py: fix for conditional skip of test

This fixes the recently introduced commit 5f7bafd0, running the skip
function instead of passing the function itself to `unittest.skipIf`.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/functional/test_output.py

@@ -266,7 +266,7 @@ class OutputPluginTest(unittest.TestCase):
             except OSError:
                 pass
 
-    @unittest.skipIf(image_output_uncapable,
+    @unittest.skipIf(image_output_uncapable(),
                      "Uncapable of generating images with PIL library")
     def test_gendata(self):
         tmpfile = tempfile.mktemp()