CMake: only add -Werror in Debug builds
CMake: only add -Werror in Debug builds

(cherry picked from commit e84dc78094abeacaef3860964cf8536c7e39a059)

JGrothoff/open62541,StalderT/open62541,JGrothoff/open62541,StalderT/open62541,open62541/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,jpfr/open62541,jpfr/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,jpfr/open62541,open62541/open62541,jpfr/open62541