perf symbols: Add missing dso.h header

This was being obtained only indirectly, by luck.

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-xeolxwr3iftwfw9kmw26shfe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>