libserdes: fix dependencies, add head
libserdes: fix dependencies, add head

libserdes depends on jansson directly for its schema registry support;
i.e., it is not just a transitive dependency via avro-c. List it
explicitly, in case avro-c one day stops depending on jansson.

Also mark that libserdes requires "curl" from macOS.

Finally, libserdes does not actually depend on librdkafka, except for
some examples that use librdkafka. Exclude it, because we don't install
those examples anyway, and then adjust the test to test libserdes
directly.

nandub/homebrew-core,mvitz/homebrew-core,aabdnn/homebrew-core,mfikes/homebrew-core,filcab/homebrew-core,lembacon/homebrew-core,Linuxbrew/homebrew-core,mbcoguno/homebrew-core,BrewTestBot/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,mvitz/homebrew-core,cblecker/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,spaam/homebrew-core,adam-moss/homebrew-core,jvillard/homebrew-core,jabenninghoff/homebrew-core,sjackman/homebrew-core,edporras/homebrew-core,JCount/homebrew-core,chrisfinazzo/homebrew-core,BrewTestBot/homebrew-core,edporras/homebrew-core,mahori/homebrew-core,jvillard/homebrew-core,jabenninghoff/homebrew-core,LinuxbrewTestBot/homebrew-core,Moisan/homebrew-core,lemzwerg/homebrew-core,stgraber/homebrew-core,mbcoguno/homebrew-core,LinuxbrewTestBot/homebrew-core,mahori/homebrew-core,makigumo/homebrew-core,makigumo/homebrew-core,rwhogg/homebrew-core,dsXLII/homebrew-core,spaam/homebrew-core,lembacon/homebrew-core,Linuxbrew/homebrew-core,zyedidia/homebrew-core,Homebrew/homebrew-core,maxim-belkin/homebrew-core,Homebrew/homebrew-core,jvillard/homebrew-core,lembacon/homebrew-core,chrisfinazzo/homebrew-core,mvitz/homebrew-core,aabdnn/homebrew-core,filcab/homebrew-core,Moisan/homebrew-core,mfikes/homebrew-core,nbari/homebrew-core,rwhogg/homebrew-core,nbari/homebrew-core,nandub/homebrew-core,lemzwerg/homebrew-core,dsXLII/homebrew-core,adam-moss/homebrew-core,JCount/homebrew-core,sjackman/homebrew-core,cblecker/homebrew-core,uyjulian/homebrew-core,maxim-belkin/homebrew-core