add lz4 to ld-config

[mikolaj-kow]

Allow to build properly. Without it:

[100%] Linking CXX executable openbmpd
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/librdkafka.a(rdkafka_lz4.o): in function `rd_kafka_lz4_decompress':
(.text+0x5a): undefined reference to `LZ4F_createDecompressionContext'