llvm-isel-fuzzer: Add link-time dependency on BitWriter

This should fix the undefined reference to WriteBitcodeToFile here:
http://bb.pgr.jp/builders/i686-mingw32-RA-on-linux/builds/31682

(Why does every different bot seem to have a different level of
finickiness about LLVM_LINK_COMPONENTS?)

llvm-svn: 312345