Replace '#ifndef QXMPP*_H' with '#pragma once'

[melvo]

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/doap.xml
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[lnjX]

I actually haven't replaced the header guards with #pragma once on purpose. #pragma once is afaik controversial and not every project using QXmpp might want to use it.

[melvo]

I actually haven't replaced the header guards with #pragma once on purpose. #pragma once is afaik controversial and not every project using QXmpp might want to use it.

@lnjX I thought using #pragma once would be the new way of doing it because you used it for the new files QXmppPubSubBaseItem.h and QXmppBlockingManager.h. What are the disadvantages?

[lnjX]

Sorry, I must have used pragma once in the BlockingManager and PubSubBaseItem by accident.

Using pragma once probably isn't a big problem, but I still think it's better to use include guards in a library like this. See e.g. https://stackoverflow.com/a/23699893