selftests/mm: include mman header to access MREMAP_DONTUNMAP identifier

Definition for MREMAP_DONTUNMAP is not present in glibc older than 2.32
thus throwing an undeclared error when running make on mm.  Including
linux/mman.h solves the build error for people having older glibc.

Link: https://lkml.kernel.org/r/20231012155257.891776-1-samasth.norway.ananda@oracle.com
Fixes: 0183d777

 ("selftests: mm: remove duplicate unneeded defines")
Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Closes: https://lore.kernel.org/linux-mm/CA+G9fYvV-71XqpCr_jhdDfEtN701fBdG3q+=bafaZiGwUXy_aA@mail.gmail.com/
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>