core: Fix missing channel unref on cached session watch
core: Fix missing channel unref on cached session watch

g_io_add_watch function takes own reference so local one needs to
be dropped. Fix following Valgrind report:

126 (120 direct, 6 indirect) bytes in 1 blocks are definitely lost in
    loss record 146 of 158
at 0x4C2745D: malloc (in /usr/lib64/valgrind/
   vgpreload_memcheck-amd64-linux.so)
by 0x4E7FE6E: g_malloc (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x4EBC5B7: g_io_channel_unix_new (in /usr/lib64/
   libglib-2.0.so.0.3800.2)
by 0x4496B4: search_completed_cb (sdp-client.c:129)
by 0x45217B: sdp_process (sdp.c:4350)
by 0x449814: search_process_cb (sdp-client.c:228)
by 0x4E7A2A5: g_main_context_dispatch (in /usr/
   lib64/libglib-2.0.so.0.3800.2)
by 0x4E7A627: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x4E7AA39: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x404372: main (main.c:550)

pstglia/external-bluetooth-bluez,silent-snowman/bluez,ComputeCycles/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,pkarasev3/bluez,mapfau/bluez,mapfau/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez