malloc: Fix mallinfo deprecation declaration

It fixes the build issue below introduced by e3960d1c (Add
mallinfo2  function that support sizes >= 4GB).  It moves the
__MALLOC_DEPRECATED to the usual place for function attributes:

  In file included from ../include/malloc.h:3,
                   from ../sysdeps/x86_64/multiarch/../../../test-skeleton.c:31,
                   from ../sysdeps/x86_64/multiarch/test-multiarch.c:96:
  ../malloc/malloc.h:118:1: error: empty declaration [-Werror]
    118 | __MALLOC_DEPRECATED;

It also adds the required deprecated warning suppression on the tests.

Checked on x86_64-linux-gnu.