[fix] Simplified compiling package without iwinfo

[pandafy]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Description of Changes

Copy iwinfo.lua only when CONFIG_NETJSON_MONITORING_IWINFO is set to true.

[coveralls]

Pull Request Test Coverage Report for Build 11725414195

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 99.597%

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Files with Coverage Reduction	New Missed Lines	%
../files/lib/openwisp-monitoring/monitoring.lua	2	85.71%

Totals
Change from base Build 11042830109:	-0.06%
Covered Lines:	3216
Relevant Lines:	3229

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💛 - Coveralls

[nemesifier]

Thanks.