find_dependency: unset cmake_fd_version variable at end of macro

This matches the other macro variables.
diff --git a/Modules/CMakeFindDependencyMacro.cmake b/Modules/CMakeFindDependencyMacro.cmake
index cf0206b..9334ba3 100644
--- a/Modules/CMakeFindDependencyMacro.cmake
+++ b/Modules/CMakeFindDependencyMacro.cmake
@@ -65,6 +65,7 @@
       set(${CMAKE_FIND_PACKAGE_NAME}_FOUND False)
       return()
     endif()
+    set(cmake_fd_version)
     set(cmake_fd_required_arg)
     set(cmake_fd_quiet_arg)
     set(cmake_fd_exact_arg)