perf symbols: Add missing linux/refcount.h to symbol.h

We use refcount_t there, so we need that header or else it'll break when
we remove dso.h, that is from where it is getting that definition now...

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-5albrk0uve6x9cf6x3ebwpae@git.kernel.org


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