make: disable '-Wdangling-pointer' warning with gcc 12

The patch is similar to what has been done in linux kernel, as this
warning effectively prevents us from adding list elements to local list
head. See https://github.com/torvalds/linux/commit/49beadbd47c2

Else we have:

    CC       criu/mount.o
  In file included from criu/include/cr_options.h:7,
                   from criu/mount.c:13:
  In function '__list_add',
      inlined from 'list_add' at include/common/list.h:41:2,
      inlined from 'mnt_tree_for_each' at criu/mount.c:1977:2:
  include/common/list.h:35:19: error: storing the address of local variable 'postpone' in
    '((struct list_head *)((char *)start + 8))[24].prev' [-Werror=dangling-pointer=]
     35 |         new->prev = prev;
        |         ~~~~~~~~~~^~~~~~
  criu/mount.c: In function 'mnt_tree_for_each':
  criu/mount.c:1972:19: note: 'postpone' declared here
   1972 |         LIST_HEAD(postpone);
        |                   ^~~~~~~~

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

Makefile

@@ -106,6 +106,15 @@ DEFINES			+= -D_GNU_SOURCE
 
 WARNINGS		:= -Wall -Wformat-security -Wdeclaration-after-statement -Wstrict-prototypes
 
+# -Wdangling-pointer results in false warning when we add a list element to
+# local list head variable. It is false positive because before leaving the
+# function we always check that local list head variable is empty, thus
+# insuring that pointer to it is not dangling anywhere, but gcc can't
+# understand it.
+# Note: There is similar problem with kernel list, where this warning is also
+# disabled: https://github.com/torvalds/linux/commit/49beadbd47c2
+WARNINGS		+= -Wno-dangling-pointer -Wno-unknown-warning-option
+
 CFLAGS-GCOV		:= --coverage -fno-exceptions -fno-inline -fprofile-update=atomic
 export CFLAGS-GCOV