[polly] Drop nonexistant LLVM_PLUGIN_EXPORT macro from llvmGetPassPluginInfo()

Fixes build:
/build/polly/lib/Support/RegisterPasses.cpp:709:80: error: expected ';' after top level declarator
extern "C" ::llvm::PassPluginLibraryInfo LLVM_ATTRIBUTE_WEAK LLVM_PLUGIN_EXPORT
                                                                               ^
                                                                              ;

Was missed in rL332796 / D47082

llvm-svn: 332814