tests: securityselinuxlabel: Add QEMU_CAPS_VNC to fake qemuCaps

In commit 45270337 forgot to make sure that tests pass.
Add the missing capability to fix the test.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

tests/securityselinuxlabeltest.c

@@ -351,6 +351,7 @@ mymain(void)
         return EXIT_FAILURE;
 
     virQEMUCapsSet(qemuCaps, QEMU_CAPS_DEVICE_CIRRUS_VGA);
+    virQEMUCapsSet(qemuCaps, QEMU_CAPS_VNC);
 
     if (qemuTestCapsCacheInsert(driver.qemuCapsCache, qemuCaps) < 0)
         return EXIT_FAILURE;