perf sort: Remove needless headers from sort.h, provide fwd struct decls

Reducing the includes hell a bit more, speeding up the build and
avoiding needless rebuilds when just one of those files gets updated.

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-u63el2vqsovsmnhebx1rcixo@git.kernel.org


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