Fix "perf tools: Synthesize GROUP_DESC feature in pipe mode" in the LT 4.14 branch

[Greensky, James J]

Commit d38d272592737ea88a20 ("perf tools: Synthesize GROUP_DESC feature in pipe mode") broke the LT 4.14 branch when using event groups in pipe-mode.

  # perf record -e '{cycles,instructions,branches}' -- sleep 4 | perf report
  # To display the perf.data header info, please use --header/--header-only options
  #
  Oxd7c [0x60]: failed to process type: 80
  Error:
  Failed to process sample

Commit a2015516c5c0be932a69 ("perf record: Synthesize features before events in pipe mode") is the fix. Can we get this cherry-picked and applied?