rpcd: add direct dependency on libjson-c

The OpenWrt buildroot ABI version rebuild tracker does not handle
transient dependencies, therefor add all libraries linked by rpcd
as direct dependencies to the corresponding binary package definition.

This ensures that rpcd is automatically rebuilt and relinked if any
of these libraries has its ABI_VERSION updated in the future.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>