Resolve unused variable in plain build

[alpha0010]

Handles compiler warning when not adding extensions:

C/C++: [...]/node_modules/@op-engineering/op-sqlite/cpp/bridge.cpp:92:9: warning: unused variable 'errMsg' [-Wunused-variable]
C/C++:    92 |   char *errMsg;
C/C++:       |         ^~~~~~
C/C++: 1 warning generated.

[ospfranco]

Harmless warning, adding a pre-processor flag makes this more complicated in the future