Fix missing target_include_directories() for gtest target.

Change-Id: I0c2f0473bcc977bda7bae2f550934d0505ee3e11

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 0b0fc7d..de4c894 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -346,6 +346,7 @@
 
 if (BUILD_TESTING AND GFLAGS)
   add_library(gtest gmock_gtest_all.cc gmock_main.cc)
+  target_include_directories(gtest PUBLIC ${Ceres_SOURCE_DIR}/internal/ceres)
   if (BUILD_SHARED_LIBS)
     # Define gtest-specific shared library flags for compilation.
     append_target_property(gtest COMPILE_DEFINITIONS