selftests/mm: guard defines from shm

thuge-gen.c defines SHM_HUGE_* macros that are provided by the uapi since
4.14.  These macros get redefined when compiling with Android's bionic
because its sys/shm.h will import the uapi definitions.

However if linux/shm.h is included, with glibc, sys/shm.h will clash on
some struct definitions:

  /usr/include/linux/shm.h:26:8: error: redefinition of ‘struct shmid_ds’
     26 | struct shmid_ds {
        |        ^~~~~~~~
  In file included from /usr/include/x86_64-linux-gnu/bits/shm.h:45,
                   from /usr/include/x86_64-linux-gnu/sys/shm.h:30:
  /usr/include/x86_64-linux-gnu/bits/types/struct_shmid_ds.h:24:8: note: originally defined here
     24 | struct shmid_ds
        |        ^~~~~~~~

For now, guard the SHM_HUGE_* defines with ifndef to prevent redefinition
warnings on Android bionic.

Link: https://lkml.kernel.org/r/20240605223637.1374969-3-edliaw@google.com


Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>