Export the required symbol from DynamicLibraryTests

Running unittests/Support/DynamicLibrary/DynamicLibraryTests fails when LLVM is
configured with LLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON, because the test's version
script only contains symbols extracted from the static libraries, that the test
links with, but not those from the main object/executable itself. The patch
explicitly exports the one symbol needed by the test.

This change fixes https://bugs.llvm.org/show_bug.cgi?id=32893

Patch authored by Momchil Velikov.

Differential Revision: https://reviews.llvm.org/D33490

llvm-svn: 303979