We need only a struct forward declaration, so prune the header
dependency tree a bit more.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-oqvgf04w4ku8xasrz79zquim@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>