package/upmpdcli: needs NPTL

The software uses pthread_mutexattr_setrobust which isn't available in
uClibc-ng Linuxthreads support.

Fixes:
 - http://autobuild.buildroot.org/results/197615ad077b0a719954024c3ba182dac43a0555



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Acked-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>