gdbus: Fix memory leak on properties_set()

gdbus: Fix memory leak on properties_set()

The pointer returned by dbus_message_iter_get_signature() must be freed
with dbus_free().

Fixes this memory leak:

==1857== 16 bytes in 1 blocks are definitely lost in loss record 104 of
251
==1857==    at 0x402BF52: realloc (in
/usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==1857==    by 0x415E286: dbus_realloc (in
/lib/i386-linux-gnu/libdbus-1.so.3.5.8)
==1857==    by 0x415E70B: ??? (in
/lib/i386-linux-gnu/libdbus-1.so.3.5.8)
==1857==    by 0x415F17B: ??? (in
/lib/i386-linux-gnu/libdbus-1.so.3.5.8)
==1857==    by 0x414CB33: dbus_message_iter_get_signature (in
/lib/i386-linux-gnu/libdbus-1.so.3.5.8)
==1857==    by 0x8053239: properties_set (object.c:899)
==1857==    by 0x5FFFFF: ???
==1857==