Adding the missing LTO API to lto.exports

Summary:
lto_codegen_set_should_embed_uselists is introduced in r235943 but not
added to lto.exports. Add to export list to expose the API.

Reviewers: dexonsmith

Subscribers: rafael, llvm-commits

Differential Revision: http://reviews.llvm.org/D10658

llvm-svn: 240442