Prefix symbols of internal libjson-c with gdal_ to avoid issues if an app links with GDAL and an external (older) libjsonc

git-svn-id: https://svn.osgeo.org/gdal/trunk@26295 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx