cmake: Fix find_library for ubusd and examples/server

cmake: Fix find_library for ubusd and examples/server

Both ubusd and cli TARGET_LINK_LIBRARIES reference ${json} which is
obtained via find_library(), but since the find_library() is searched
after the TARGET_LINK_LIBRARIES for ubusd, ubusd always gets an empty
${json} variable.

examples/server also links against libjson-c, but we were not setting
TARGET_LINK_LIBRARIES accordingly, so do that too with ${json} appended.

This was causing linking errors for ubusd and then examples/server using
an external toolchain (stbgcc-4.8-1.x).

Fixes: 9f52d1769b762 ("cli: use the new json-c library name")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>