Import Threads::Threads CMake target in CeresConfig.cmake

- Required now that Threads is a public dependency for Ceres and so
  Threads::Threads will appear in the dependency list for the Ceres
  target and so must be defined when Ceres itself is imported.

Change-Id: Ia5f8ea16e8f24cf0158f40e1fc8d3962106459a6

cmake/CeresConfig.cmake.in

diff --git a/cmake/CeresConfig.cmake.in b/cmake/CeresConfig.cmake.in
index 009e32f..0ef7021 100644
--- a/cmake/CeresConfig.cmake.in
+++ b/cmake/CeresConfig.cmake.in
@@ -161,6 +161,9 @@
 # Set the version.
 set(CERES_VERSION @CERES_VERSION@ )
 
+include(CMakeFindDependencyMacro)
+find_dependency(Threads)
+
 # Eigen.
 # Flag set during configuration and build of Ceres.
 set(CERES_EIGEN_VERSION @EIGEN_VERSION@)