perf tools: Fix accidentally preprocessed snprintf callback

struct sort_entry has a callback named snprintf that turns an
entry into a string result.
But there are glibc versions that implement snprintf through a
macro. The following expression is then going to get the snprintf
call preprocessed:

        ent->snprintf(...)

to finally end up in a build error:

        util/hist.c: Dans la fonction «hist_entry__snprintf» :
        util/hist.c:539: erreur: «struct sort_entry» has no member named «__builtin___snprintf_chk»

To fix this, prepend struct sort_entry callbacks with an "se_"
prefix.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>