perf tests bp_account: Fix build with clang-6

perf tests bp_account: Fix build with clang-6

To shut up this compiler warning:

    CC       /tmp/build/perf/tests/bp_account.o
    CC       /tmp/build/perf/tests/task-exit.o
    CC       /tmp/build/perf/tests/sw-clock.o
  tests/bp_account.c:106:20: error: pointer type mismatch ('int (*)(void)' and 'void *') [-Werror,-Wpointer-type-mismatch]
          void *addr = is_x ? test_function : (void *) &the_var;
                            ^ ~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~
  1 error generated.

Noticed with clang 6 on fedora rawhide.

  [perfbuilder@44490f0e7241 perf]$ clang -v
  clang version 6.0.0 (tags/RELEASE_600/final)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin
  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/8
  Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/8
  Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/8
  Candidate multilib: .;@m64
  Candidate multilib: 32;@m32
  Selected multilib: .;@m64
  [perfbuilder@44490f0e7241 perf]$

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 032db28e5fa3 ("perf tests: Add breakpoint accounting/modify test")
Link: https://lkml.kernel.org/n/tip-a3jnkzh4xam0l954de5tn66d@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>