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 <romain.naour@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-devtools/poke/poke_1.2.bb

@@ -24,8 +24,9 @@ acpaths = "-I ./m4"
 
 EXTRA_OECONF = "--disable-gui \
                 --disable-libnbd \
-                --disable-mi \
                 --with-libreadline-prefix=${STAGING_INCDIR} \
                 "
 
+PACKAGECONFIG[mi] = "--enable-mi,--disable-mi,json-c"
+
 FILES_${PN} += "${datadir}/emacs/site-lisp"