selftests/mm: fix khugepaged_race build with older glibc

glibc versions before 2.37 do not define MADV_COLLAPSE in <sys/mman.h>, so
khugepaged_race fails to build against them (e.g. glibc 2.34):

[root@localhost mm]# make
  CC       khugepaged_race
khugepaged_race.c: In function 'main':
khugepaged_race.c:506:23: error: 'MADV_COLLAPSE' undeclared (first use in this function); did you mean 'MADV_COLD'?
  506 |       hpage_pmd_size, MADV_COLLAPSE);
      |                       ^~~~~~~~~~~~~
      |                       MADV_COLD
khugepaged_race.c:506:23: note: each undeclared identifier is reported only once for each function it appears in
make: *** [../lib.mk:225: /data/code/mm/tools/testing/selftests/mm/khugepaged_race] Error 1

Include <linux/mman.h> to get the UAPI definition of MADV_COLLAPSE.

Link: https://lore.kernel.org/20261027122013.4286-1-zenghui.yu@linux.dev
Fixes: 66c3744644 ("selftests/mm: add khugepaged race harness")
Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>

tools/testing/selftests/mm/khugepaged_race.c

@@ -8,6 +8,7 @@
 #define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
+#include <linux/mman.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>