Fix flag -DJSONCPP_USE_SECURE_MEMORY:BOOL=TRUE

[tfc]

The Cmake description uses JSONCPP_USE_SECURE_MEMORY and the source uses JSONCPP_USING_SECURE_MEMORY.

Is it known that the tests don't compile with this setting being enabled?

[baylesj]

Thanks for catching this. I'll look at adding a new bot configuration that would catch this.