Fix Qt build on OSX

Compiling boost::interprocess::message_queue against
boost 1.50 macports with -arch i386 (how releases are built,
for minimum download size and maximum compatibility) is failing:

src/qt/qtipcserver.cpp:37: error: no matching function for call to ‘boost::interprocess::message_queue_t<boost::interprocess::offset_ptr<void, int, long unsigned int, 0u> >::timed_receive(char (*)[257], long unsigned int, size_t&, unsigned int&, boost::posix_time::ptime&)’

This is probably a boost or macports bug, but since interprocess::message_queue
is only used for URI support, which isn't implemented on OSX anyway, I fixed
the build by #ifdef'ing out that code.