Fix memory leak
Fix memory leak

This fixes the memory leak below:

	$ sudo valgrind -v --leak-check=full fwupdtool get-devices
	(...)
	==3244345== 133 (64 direct, 69 indirect) bytes in 2 blocks are definitely lost in loss record 2,488 of 2,681
	==3244345==    at 0x4845899: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
	==3244345==    by 0x4C04D59: g_malloc (in /usr/lib/libglib-2.0.so.0.7000.4)
	==3244345==    by 0x4C1C816: g_slice_alloc (in /usr/lib/libglib-2.0.so.0.7000.4)
	==3244345==    by 0x4C1CE9E: g_slice_alloc0 (in /usr/lib/libglib-2.0.so.0.7000.4)
	==3244345==    by 0x4B84684: g_type_create_instance (in /usr/lib/libgobject-2.0.so.0.7000.4)
	==3244345==    by 0x4B6BB65: ??? (in /usr/lib/libgobject-2.0.so.0.7000.4)
	==3244345==    by 0x4B6CAF4: g_object_new_with_properties (in /usr/lib/libgobject-2.0.so.0.7000.4)
	==3244345==    by 0x4B6D659: g_object_new (in /usr/lib/libgobject-2.0.so.0.7000.4)
	==3244345==    by 0x4AA8969: ??? (in /usr/lib/libgio-2.0.so.0.7000.4)
	==3244345==    by 0x153967: fu_engine_load_local_metadata_watches (fu-engine.c:7050)
	==3244345==    by 0x1540FC: fu_engine_load (fu-engine.c:7230)
	==3244345==    by 0x124D29: fu_util_start_engine (fu-tool.c:262)
	(...)
	==3244345== LEAK SUMMARY:
	==3244345==    definitely lost: 64 bytes in 2 blocks
	==3244345==    indirectly lost: 69 bytes in 2 blocks
	==3244345==      possibly lost: 1,936 bytes in 8 blocks
	==3244345==    still reachable: 234,668 bytes in 3,072 blocks
	==3244345==         suppressed: 0 bytes in 0 blocks
	==3244345== Reachable blocks (those to which a pointer was found) are not shown.
	==3244345== To see them, rerun with: --leak-check=full --show-leak-kinds=all
	==3244345==
	==3244345== ERROR SUMMARY: 9 errors from 9 contexts (suppressed: 0 from 0)

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd