Always define rd_ut_coverage_check

[StephanDollberg]

Define rd_ut_coverage_check even in non-devel mode as the lds-gen.py script adds the symbol unconditionally.

Otherwise LLD complains about ld:

error: version script assignment of 'global' to symbol 'rd_ut_coverage_check' failed: symbol not defined

Fixes https://github.com/confluentinc/librdkafka/issues/4593

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