memblock tests: define MIGRATE_CMA

kho_scratch_migratetype(), defined in include/linux/memblock.h uses enum
migratetype. This breaks build for memblock tests with:

./linux/memblock.h:634:73: error: parameter 2 (‘mt’) has incomplete type
  634 |                                                        enum migratetype mt)

Fix it by defining enum migratetype and MIGRATE_CMA. As is the case with
the other headers in tools/testing/memblock, do not bring in the whole
thing, only what is needed.

Reported-by: Mike Rapoport <rppt@kernel.org>
Closes: https://lore.kernel.org/linux-mm/afcdDm4aAJvNaQqH@kernel.org/
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Link: https://patch.msgid.link/20260504102742.3833159-1-pratyush@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: prati0100

tools/testing/memblock/linux/mmzone.h

@@ -35,4 +35,8 @@ typedef struct pglist_data {
 
 } pg_data_t;
 
+enum migratetype {
+	MIGRATE_CMA,
+};
+
 #endif