bcachefs: Fix build error on weird gcc

fixes
./include/linux/stddef.h:8:14: error: positional initialization of field in ‘struct’ declared with ‘designated_init’ attribute [-Werror=designated-init]
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super-io.c

@@ -1487,9 +1487,7 @@ static const struct bch_sb_field_ops *bch2_sb_field_ops[] = {
 #undef x
 };
 
-static const struct bch_sb_field_ops bch2_sb_field_null_ops = {
-	NULL
-};
+static const struct bch_sb_field_ops bch2_sb_field_null_ops;
 
 static const struct bch_sb_field_ops *bch2_sb_field_type_ops(unsigned type)
 {