To reduce the header dependency and avoid unnecessary rebuilds when
things change in symbol.h.

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-6duflwliprh2tr47w5x4t260@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>