bitset: fix compilation on macOS 12

Asserts are disabled in the Release build, that leads to:

$ cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_FLAGS=-Werror
$ make

[ 43%] Building C object src/lib/bitset/CMakeFiles/bitset.dir/bitset.c.o
src/lib/bitset/bitset.c:169:9: error: variable 'cardinality_check' set but not used [-Werror,-Wunused-but-set-variable]
        size_t cardinality_check = 0;
               ^

NO_DOC=build fix
NO_TEST=build fix
NO_CHANGELOG=build fix

src/lib/bitset/bitset.c

@@ -174,6 +174,7 @@ tt_bitset_info(struct tt_bitset *bitset, struct tt_bitset_info *info)
 		page = tt_bitset_pages_next(&bitset->pages, page);
 	}
 
+	(void)cardinality_check;
 	assert(tt_bitset_cardinality(bitset) == cardinality_check);
 }