memblock tests: fix undefined reference to `BIT'

commit 772dd034 ("mm: enumerate all gfp flags") define gfp flags
with the help of BIT, while gfp_types.h doesn't include header file for
the definition. This through an error on building memblock tests.

Let's include linux/bits.h to fix it.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Suren Baghdasaryan <surenb@google.com>
CC: Michal Hocko <mhocko@suse.com>
Link: https://lore.kernel.org/r/20240402132701.29744-4-richard.weiyang@gmail.com


Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>