Fix uninitialized CMake variable in version.in

CMake's project() command sets jsoncpp_VERSION (lowercase prefix),
not JSONCPP_VERSION. The wrong variable caused the generated version
file to be empty.

Fixes #1656

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: baylesj

version.in

@@ -1 +1 @@
-@JSONCPP_VERSION@
+@jsoncpp_VERSION@