net/librdkafka: work around undefined version symbol error

When building outside of poudriere, net/librdkafka can fail to link,
with an error similar to:

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

Work around this by adding -Wl,--undefined-version to LDFLAGS.

While here, pet portlint.

PR: 275866
Approved by: maintainer timeout (2 weeks)
MFH: 2024Q4