pkg: Add check for php-json

[arshad512]

php-json package is used by webgui. While the build would pass if php-json package is missing. It may not render the graphs at all.

This patch adds check for php-json. During the time of configure. It does not change workflow or alters functions. If php-json is missing it will throw an "warning" under config.log so user can review it post running and add that if necessary. Hard requirement of php-json was not selected as not all users would like to use webgui. Therefore the changes under robinhood.spec.in ("BuildRequires:") was not chosen.

Change-Id: I45c3aeccdd4983a5ae9935dc383b55ced9ffbc5e

[tl-cea]

Isn't this something to be added in the %Requires section of the robinhood-webgui package?

I don't get why requiring this package at build time would affect the rendering on the system where the generated RPM is finally installed.