transmission: fix build when using system libminiupnpc with libutp disabled

Fixes:

  http://autobuild.buildroot.net/results/63a3caac7e1e7b5ab268d9be3cd77617df619659/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>