libcontainer: fix integration failure in "make test"

When running inside a Docker container, systemd is not available. The
new TestFdLeaksSystemd forgot to include the relevant t.Skip section.

Fixes: a7feb42395 ("libct/int: add TestFdLeaksSystemd")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/integration/exec_test.go

@@ -1921,6 +1921,9 @@ func TestFdLeaks(t *testing.T) {
 }
 
 func TestFdLeaksSystemd(t *testing.T) {
+	if !systemd.IsRunningSystemd() {
+		t.Skip("Test requires systemd.")
+	}
 	testFdLeaks(t, true)
 }