obexd/server: Fix leaking drivers list

obexd/server: Fix leaking drivers list

The leak can be detected by using G_SLICE=always-malloc which will
produce the following trace using valgrind:
112 bytes in 7 blocks are definitely lost in loss record 123 of 167
   at 0x4A06409: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x3B03C4D89E: g_malloc (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x3B03C6344D: g_slice_alloc (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x3B03C647A5: g_slist_append (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x424DD3: obex_service_driver_list (service.c:76)
   by 0x42517F: obex_server_init (server.c:64)
   by 0x40D439: main (main.c:304)