perf test: Fix a build error in x86 topdown test

perf test: Fix a build error in x86 topdown test

There's an environment that caused the following build error.  Include
"debug.h" (under util directory) to fix it.

  arch/x86/tests/topdown.c: In function 'event_cb':
  arch/x86/tests/topdown.c:53:25: error: implicit declaration of function 'pr_debug'
                                         [-Werror=implicit-function-declaration]
     53 |                         pr_debug("Broken topdown information for '%s'\n", evsel__name(evsel));
        |                         ^~~~~~~~
  cc1: all warnings being treated as errors

Link: https://lore.kernel.org/r/20250815164122.289651-1-namhyung@kernel.org
Fixes: 5b546de9cc177936 ("perf topdown: Use attribute to see an event is a topdown metic or slots")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>