Fix uninitialized CMake variable in version.in (#1658)

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>

version.in

diff --git a/version.in b/version.in
index bfc03f7..f1e3335 100644
--- a/version.in
+++ b/version.in
@@ -1 +1 @@
-@JSONCPP_VERSION@
+@jsoncpp_VERSION@