build(jxl): Use correct cmake variables for the include directories (#4813)

There was feedback[1] in the prior commit that the changes were a bit
atypical when it comes to cmake conventions.

This is a partial revert of 5ca82dc to
better align with conventions but still fix the problem.

[1]
#4810 (comment)


Signed-off-by: Jesse Yurkovich <[email protected]>

Author: jessey-git

src/cmake/modules/FindJXL.cmake

@@ -41,4 +41,5 @@ find_package_handle_standard_args(JXL
 
 if(JXL_FOUND)
   set(JXL_LIBRARIES ${JXL_LIBRARY} ${JXL_THREADS_LIBRARY})
+  set(JXL_INCLUDES ${JXL_INCLUDE_DIR})
 endif(JXL_FOUND)

src/jpegxl.imageio/CMakeLists.txt

@@ -4,7 +4,7 @@
 
 if (JXL_FOUND)
     add_oiio_plugin (jxlinput.cpp jxloutput.cpp
-                     INCLUDE_DIRS ${JXL_INCLUDE_DIR}
+                     INCLUDE_DIRS ${JXL_INCLUDES}
                      LINK_LIBRARIES ${JXL_LIBRARIES}
                      DEFINITIONS "USE_JXL")
 else()