qt: Fix out-of-tree GUI builds

Without this patch:

• When I compile the GUI from the bitcoin directory itself, it works as expected.

• When I build the GUI in an out-of-tree build, I cannot get it to select tabs. When I click, say the "Receive" tab nothing happens, the button selects but it doesn't switch the page. The rest - even the debug window - seems to work.

See full discussion here:
https://github.com/bitcoin/bitcoin/pull/7911#issuecomment-212413442

This turned out to be caused by a mismatch in the arguments to moc,
preventing it from finding bitcoin-config.h. Fix this by passing
$(DEFAULT_INCLUDES) to it, which gets set to the appropriate
path by autoconf itself.