azmq: add NPTL dependency

boost-log depends on BR2_TOOLCHAIN_HAS_THREADS_NPTL so add this
dependency to azmq

Fixes:
 - http://autobuild.buildroot.net/results/ffa5f21d7e7c38ea7adebc84f1cc8ee4cff74f1b



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