build: Use Autoconf macro @includedir@ for jansson.pc.
build: Use Autoconf macro @includedir@ for jansson.pc.

This is more consistent with what is done for other pkg-config
variables.

markalanj/jansson,markalanj/jansson,bstarynk/jansson,bstarynk/jansson,AmesianX/jansson,akheron/jansson,akheron/jansson,akheron/jansson,Mephistophiles/jansson,AmesianX/jansson,markalanj/jansson,Mephistophiles/jansson,bstarynk/jansson