Fixed a few unused variables.

src/tests/libknot/libknot/zone_tree_tests.c

src/tests/libknot/realdata/libknot/rdata_tests_realdata.c

src/tests/libknot/realdata/libknot_tests_loader_realdata.c

src/zcompile/parser-util.c

src/zcompile/zcompile_main.c