perf build: Fix a build error on REFCNT_CHECKING=1

perf build: Fix a build error on REFCNT_CHECKING=1

Recently it added -fno-strict-aliasing to sync with the kernel behavior.
But it caused an error due to potential uninitialized access like below:

  In file included from util/symbol.c:27:
  In function ‘dso__set_symbol_names_len’,
      inlined from ‘dso__sort_by_name’ at util/symbol.c:638:4:
  util/dso.h:654:46: error: ‘len’ may be used uninitialized [-Werror=maybe-uninitialized]
    654 |         RC_CHK_ACCESS(dso)->symbol_names_len = len;
        |                                              ^
  util/symbol.c: In function ‘dso__sort_by_name’:
  util/symbol.c:634:24: note: ‘len’ was declared here
    634 |                 size_t len;
        |                        ^~~

Let's just initialize it with 0.

Fixes: 55a18d2f3ff79c90 ("perf build: enable -fno-strict-aliasing")
Closes: https://lore.kernel.org/r/aF7JC8zkG5-_-nY_@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>