perf script: Move map__fprintf_srccode() to near its only user

No need to have it elsewhere.

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-8cw846pudpxo0xdkvi9qnvrh@git.kernel.org


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