perf bench futex: Fix prctl include in musl libc

Namhyung Kim reported:

  I've updated the perf-tools-next to v6.16-rc1 and found a build error
  like below on alpine linux 3.18.

    In file included from bench/futex.c:6:
    /usr/include/sys/prctl.h:88:8: error: redefinition of 'struct prctl_mm_map'
       88 | struct prctl_mm_map {
          |        ^~~~~~~~~~~~
    In file included from bench/futex.c:5:
    /linux/tools/include/uapi/linux/prctl.h:134:8: note: originally defined here
      134 | struct prctl_mm_map {
          |        ^~~~~~~~~~~~
    make[4]: *** [/linux/tools/build/Makefile.build:86: /build/bench/futex.o] Error 1

  git bisect says it's the first commit introduced the failure.

So both /usr/include/sys/prctl.h and /linux/tools/include/uapi/linux/prctl.h
provide struct prctl_mm_map but their include guard must be different.

/usr/include/sys/prctl.h provided by glibc contains the
prctl() declaration. It includes also linux/prctl.h.

The /usr/include/sys/prctl.h on alpine linux is different. This is
probably coming from musl. It contains the PR_* definition and the
prctl() declaration.  So it clashes here because now the one struct is
available twice.

The man page for prctl(2) says:

|       #include <linux/prctl.h>  /* Definition of PR_* constants */
|       #include <sys/prctl.h>

so musl doesn't follow this.

So don't include linux/prctl.h explicitely and add some new defines
needed if they aren't available.

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Namhyung Kim <namhyung@kernel.org>
Closes: https://lore.kernel.org/r/20250611092542.F4ooE2FL@linutronix.de
Link: https://www.openwall.com/lists/musl/2025/06/12/11


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