perf symbols: Fix dso__fprintf_symbols_by_name() to return the number of printed chars

The 'ret' variable was initialized to zero but then it was not updated
from the fprintf() return, fix it.

Reported-by: Yang Li <yang.lee@linux.alibaba.com>
cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
cc: Ingo Molnar <mingo@redhat.com>
cc: Jiri Olsa <jolsa@redhat.com>
cc: Mark Rutland <mark.rutland@arm.com>
cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Fixes: 90f18e63 ("perf symbols: List symbols in a dso in ascending name order")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>