Fix DocumentFixture.Serialization failing test on OSX

DocumentFixture.Serialization test in openstudio_utilities_tests fails
on OSX and results in the following message ...

[ RUN      ] DocumentFixture.Serialization
[openstudio.Document] <1> Boost archive exception while writing Document
out to boost serialization text format (.osd). Code 2. Message:
unregistered class - derived class not registered or exported
Segmentation fault: 11

This message means that the class needs to be exported aka BOOST_CLASS_EXPORT().

From the code this seems to be limited to OSX only.

@macumber
I don't know from a historical perspective why the #ifndef __APPLE__ but it doesn't seem necessary now.

@axelstudios @kbenne - check and make sure this builds on your Macs and the test passes with this change.