pkg-config: Include required libraries in Libs.private

In attempting to statically link against libavro, I kept running into minor-but-annoying issues, like not linking with jansson.

Currently `pkg-config --libs avro-c` and `pkg-config --libs --static avro-c` both return the same thing;  to make static linking easier, the latter should also add `-lz -lzma -lsnappy -ljansson -lpthread`, plus/minus whichever were actually used.