uhttpd: add direct dependency on libjson-c

uhttpd: add direct dependency on libjson-c

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

This ensures that uhttpd 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>
(cherry picked from commit a95ddaba0272f254a3aeade3aa0c086e1625d672)