perf symbols: Move symsrc prototypes to a separate header

So that we can remove dso.h from symbol.h and reduce the header
dependency tree.

Fixup cases where struct dso guts are needed but were obtained via
symbol.h, indirectly.

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-ip683cegt306ncu3gsz7ii21@git.kernel.org


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