perf test bpf: Address error about non-null argument for epoll_pwait 2nd arg

First noticed on Fedora Rawhide:

  tests/bpf.c: In function ‘epoll_pwait_loop’:
  tests/bpf.c:36:17: error: argument 2 null where non-null expected [-Werror=nonnull]
     36 |                 epoll_pwait(-(i + 1), NULL, 0, 0, NULL);
        |                 ^~~~~~~~~~~
  In file included from tests/bpf.c:5:
  /usr/include/sys/epoll.h:134:12: note: in a call to function ‘epoll_pwait’ declared ‘nonnull’
    134 | extern int epoll_pwait (int __epfd, struct epoll_event *__events,
        |            ^~~~~~~~~~~

  [perfbuilder@27cfe44d67ed perf-6.5.0-rc2]$ gcc -v
  Using built-in specs.
  COLLECT_GCC=gcc
  COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/13/lto-wrapper
  OFFLOAD_TARGET_NAMES=nvptx-none
  OFFLOAD_TARGET_DEFAULT=1
  Target: x86_64-redhat-linux
  Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,m2,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --enable-libstdcxx-backtrace --with-libstdcxx-zoneinfo=/usr/share/zoneinfo --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --with-isl=/builddir/build/BUILD/gcc-13.2.1-20230728/obj-x86_64-redhat-linux/isl-install --enable-offload-targets=nvptx-none --without-cuda-driver --enable-offload-defaulted --enable-gnu-indirect-function --enable-cet --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux --with-build-config=bootstrap-lto --enable-link-serialization=1
  Thread model: posix
  Supported LTO compression algorithms: zlib zstd
  gcc version 13.2.1 20230728 (Red Hat 13.2.1-1) (GCC)
  [perfbuilder@27cfe44d67ed perf-6.5.0-rc2]$

Just add that argument to address this compiler warning.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/ZMj8+bvN86D0ZKiB@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>