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

include/linux/gfp_types.h

@@ -2,6 +2,8 @@
 #ifndef __LINUX_GFP_TYPES_H
 #define __LINUX_GFP_TYPES_H
 
+#include <linux/bits.h>
+
 /* The typedef is in types.h but we want the documentation here */
 #if 0
 /**