Allow nwfilter functions to be compiled with C++

Unfortunately the NWFilter functions were outside of the
"extern C { ... }" declaration in include/libvirt/libvirt.h.in,
which means that they couldn't be properly used with C++.  Move
them inside of the braces, which should fix the problem.
Signed-off-by: NChris Lalancette <clalance@redhat.com>

include/libvirt/libvirt.h.in

@@ -2212,10 +2212,6 @@ int virConnectDomainEventRegisterAny(virConnectPtr conn,
 int virConnectDomainEventDeregisterAny(virConnectPtr conn,
                                        int callbackID);
 
-#ifdef __cplusplus
-}
-#endif
-
 
 /**
  * virNWFilter:
@@ -2279,4 +2275,8 @@ int                     virNWFilterGetUUIDString (virNWFilterPtr nwfilter,
 char *                  virNWFilterGetXMLDesc    (virNWFilterPtr nwfilter,
                                                   int flags);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __VIR_VIRLIB_H__ */