Fix exported gflags include path.

- In practice, this was not causing any errors, as
  GFLAGS_INCLUDE_DIR_HINTS is set from GFLAGS_INCLUDE_DIRS so the two
  would have been equivalent, but we should nonetheless be using the
  one set after the find_package() call.
- Note that this would only have been active for older gflags versions
  that were not built with CMake.

Change-Id: Ie54145f522c974b7685052aafb1d2f6055d22337

cmake/CeresConfig.cmake.in

diff --git a/cmake/CeresConfig.cmake.in b/cmake/CeresConfig.cmake.in
index 9fc6971..957b99d 100644
--- a/cmake/CeresConfig.cmake.in
+++ b/cmake/CeresConfig.cmake.in
@@ -313,7 +313,7 @@
         "dependency: gflags. Searched using GFLAGS_INCLUDE_DIR_HINTS: "
         "${GFLAGS_INCLUDE_DIR_HINTS} and gflags_DIR: ${gflags_DIR}.")
     endif()
-    list(APPEND CERES_INCLUDE_DIRS ${GFLAGS_INCLUDE_DIR_HINTS})
+    list(APPEND CERES_INCLUDE_DIRS ${GFLAGS_INCLUDE_DIRS})
   endif()
 endif (CERES_USES_MINIGLOG)