build(jxl): Use correct cmake variables for the include directories (AcademySoftwareFoundation#4810)

I'm unsure how this could have worked correctly. There were three
different variables used before, now there is only one.

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

Author: jessey-git

src/cmake/modules/FindJXL.cmake

@@ -41,5 +41,4 @@ 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_DIRS}
+                     INCLUDE_DIRS ${JXL_INCLUDE_DIR}
                      LINK_LIBRARIES ${JXL_LIBRARIES}
                      DEFINITIONS "USE_JXL")
 else()