Fix CMake warning about cJSON name mismatch

CMake Warning (dev) at /usr/share/cmake-3.19/Modules/FindPackageHandleStandardArgs.cmake:424 (message):
  The package name passed to `find_package_handle_standard_args` (CJSON) does
  not match the name of the calling package (cJSON).  This can lead to
  problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
  cmake/FindcJSON.cmake:23 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:125 (FIND_PACKAGE)
This warning is for project developers.  Use -Wno-dev to suppress it.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>