See https://msdn.microsoft.com/en-us/library/e821azx1.aspx
Some export symbols should be PRIVATE

Also, don't export DLLMain. It is internal function
used by CRT, and not intended to be exported.

Change-Id: I48139b985aa2ee499c2005d8b0d966a2cc6dcd0c
Reviewed-on: https://gerrit.libreoffice.org/20419Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Matúš Kukan <matus@libreoffice.org>