package/wavemon: remove NPTL dependency

wavemon does not use pthread_mutexattr_setrobust since version 0.8.2 and
https://github.com/uoaerg/wavemon/commit/d271685e03bc896829d8cc01ca0de28cd7b6ba72



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>