[dbus] fix introspect.xml for AddOnMeshPrefix()

[superwhd]

Fix the error calling AddOnMeshPrefix #2079 (comment). The interface in introspect.xml has been out-of-sync for.

Verification: adding a prefix.

$ sudo gdbus call --system --dest io.openthread.BorderRouter.wpan0 --method=io.openthread.BorderRouter.AddOnMeshPrefix --object-path /io/openthread/BorderRouter/wpan0 "(([0xfd,0x11,0x12,0x13,0x13,0x14,0x15,0x16], 64),0x5343, 0x01, false,false,false,false,false,false,false,false,false)"

()

> netdata show
Prefixes:
fd11:1213:1314:1516::/64 high d800
Routes:
Services:
44970 01 1f000500000e10 s d800
Contexts:
fd11:1213:1314:1516::/64 1 c
Commissioning:
12688 - - -
Done

[morningboata]

Please have a local test to make sure it works as expected.

[superwhd]

Please have a local test to make sure it works as expected.

Updated in the PR description.