[libfuzzer] Fix a test's run line

r342698 removed the 'UNSUPPORTED: windows' flags from a number of fuzzer
tests, however too many lines were removed from 'fuzzer-flags.test',
including the run-line that generates the test binary, which breaks that
test for me (and the clang-x64-ninja-win7 buildbot). This patch just
re-adds that line.

llvm-svn: 342720