FindDevIL: Make the ILUT library optional

Some distributions (such as ArchLinux) have only the IL and ILU
libraries and since these are mainly used, the module should succeed
even though ILUT was not found.  Removed it from the FPHSA() macro call,
making it effectively optional.

Modules/FindDevIL.cmake

@@ -69,4 +69,4 @@ find_library(ILU_LIBRARIES
 
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(DevIL DEFAULT_MSG
                                   IL_LIBRARIES ILU_LIBRARIES
-                                  ILUT_LIBRARIES IL_INCLUDE_DIR)
+                                  IL_INCLUDE_DIR)