cjson: change source to svn repo

The original cJSON submittal downloaded a .zip file from SourceForge.
The .zip file did not have a version number, making it impossible
to gaurantee that the same archive is downloaded on any given build.
It also required a custom EXTRACT command.

This patch changes the source for cJSON to the svn repository
listed on the sf project web page - giving us an actual version
number, and letting us use buildroot's normal extraction functionality.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>