perf libunwind: Fixup conversion perf_sample->user_regs to a pointer

perf libunwind: Fixup conversion perf_sample->user_regs to a pointer

The dc6d2bc2d893a878 ("perf sample: Make user_regs and intr_regs optional") misses
the changes to a file, resulting in this problem:

  $ make LIBUNWIND=1 -C tools/perf O=/tmp/build/perf-tools-next install-bin
  <SNIP>
    CC      /tmp/build/perf-tools-next/util/unwind-libunwind-local.o
    CC      /tmp/build/perf-tools-next/util/unwind-libunwind.o
  <SNIP>
  util/unwind-libunwind-local.c: In function ‘access_mem’:
  util/unwind-libunwind-local.c:582:56: error: ‘ui->sample->user_regs’ is a pointer; did you mean to use ‘->’?
    582 |         if (__write || !stack || !ui->sample->user_regs.regs) {
        |                                                        ^
        |                                                        ->
  util/unwind-libunwind-local.c:587:38: error: passing argument 2 of ‘perf_reg_value’ from incompatible pointer type [-Wincompatible-pointer-types]
    587 |         ret = perf_reg_value(&start, &ui->sample->user_regs,
        |                                      ^~~~~~~~~~~~~~~~~~~~~~
        |                                      |
        |                                      struct regs_dump **
<SNIP>
  ⬢ [acme@toolbox perf-tools-next]$ git bisect bad
  dc6d2bc2d893a878e7b58578ff01b4738708deb4 is the first bad commit
  commit dc6d2bc2d893a878e7b58578ff01b4738708deb4 (HEAD)
  Author: Ian Rogers <irogers@google.com>
  Date:   Mon Jan 13 11:43:45 2025 -0800

      perf sample: Make user_regs and intr_regs optional

Detected using:

  make -C tools/perf build-test

Fixes: dc6d2bc2d893a878 ("perf sample: Make user_regs and intr_regs optional")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250313033121.758978-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>