package/open62541: fix build without threads

Fixes:
 - http://autobuild.buildroot.org/results/87ec3c987c991f790757276316d6caf81e635fa2

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