poke: add optional json-c dependency
poke: add optional json-c dependency

From [1]:
"The poke machine-interface uses a protocol to communicate with
clients, which is based on JSON.  poke uses the json-c library to
parse and build JSON strings.
If this library is not found, poke will build without machine-interface
support."

[1] https://git.savannah.nongnu.org/cgit/poke.git/tree/DEPENDENCIES?h=releases/poke-1.2#n55

Signed-off-by: Romain Naour <48b9fcabb497dca2879ac6d9e447e6704661c558@gmail.com>
Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

openembedded/meta-openembedded,schnitzeltony/meta-openembedded,openembedded/meta-openembedded,moto-timo/meta-openembedded,openembedded/meta-openembedded,moto-timo/meta-openembedded,openembedded/meta-openembedded,moto-timo/meta-openembedded,moto-timo/meta-openembedded,schnitzeltony/meta-openembedded,schnitzeltony/meta-openembedded,openembedded/meta-openembedded,rehsack/meta-openembedded,schnitzeltony/meta-openembedded,openembedded/meta-openembedded,schnitzeltony/meta-openembedded,rehsack/meta-openembedded,rehsack/meta-openembedded,rehsack/meta-openembedded,schnitzeltony/meta-openembedded,rehsack/meta-openembedded,rehsack/meta-openembedded,rehsack/meta-openembedded,moto-timo/meta-openembedded,openembedded/meta-openembedded,openembedded/meta-openembedded,schnitzeltony/meta-openembedded