build: Fix build warning on libvirt-python

On compiling libvirt-python, we get such a warning:

libvirt-qemu-override.c: In function ‘libvirt_qemu_virConnectDomainQemuMonitorEventRegister’:
libvirt-qemu-override.c:304: warning: suggest explicit braces to avoid ambiguous ‘else’

Py_DECREF is a macro using if/else on older Python releases.
The solution is to add braces.
Python 2.7 and newer has the macro wrapped in a do { } while(0) block.

Signed-off-by: Mo Yuxiang <Moyuxiang@huawei.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>

libvirt-qemu-override.c

@@ -301,8 +301,9 @@ libvirt_qemu_virConnectDomainQemuMonitorEventRegister(PyObject *self ATTRIBUTE_U
                                                    flags);
     LIBVIRT_END_ALLOW_THREADS;
 
-    if (ret < 0)
+    if (ret < 0) {
         Py_DECREF(pyobj_cbData);
+    }
 
     py_retval = libvirt_intWrap(ret);
     return py_retval;