package/open62541: disable -Werror

Disable -Werror to avoid the following build failures raised since bump
to version 1.2.2 in commit b436b824:

/tmp/instance-0/output-1/build/open62541-v1.2.2/arch/network_tcp.c: In function 'connection_recv':
/tmp/instance-0/output-1/build/open62541-v1.2.2/arch/network_tcp.c:96:5: error: conversion to 'unsigned int' from 'int' may change the sign of the result [-Werror=sign-conversion]
   96 |     UA_fd_set(connection->sockfd, &fdset);
      |     ^~~~~~~~~

/tmp/instance-6/output-1/build/open62541-v1.2.2/plugins/ua_pubsub_udp.c: In function 'UA_PubSubChannelUDPMC_receive':
/tmp/instance-6/output-1/build/open62541-v1.2.2/plugins/ua_pubsub_udp.c:477:21: error: conversion to '__suseconds_t' {aka 'int'} from 'UA_UInt32' {aka 'unsigned int'} may change the sign of the result [-Werror=sign-conversion]
  477 |     tmptv.tv_usec = (long int)(timeout % 1000000);
      |                     ^

Fixes:
 - http://autobuild.buildroot.org/results/911811de81d8abb2a31feb8f27af1592641c6fbc
 - http://autobuild.buildroot.org/results/f0187b3f2d62e955fddeef4e90f84ba4fd642bd2



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>