make: fix dedup USEMODULE_INCLUDES in Makefile.include

    Fixes a regression introduces #9479 which breaks building on macOS.
    The introduces `uniq` function is never called, hence the includes
    ie. USEMODULE_INCLUDES are not used.

Makefile.include

@@ -329,7 +329,7 @@ $(info $(USEPKG:%=$(RIOTPKG)/%/Makefile.include))
 
 # Deduplicate includes without sorting them
 # see https://stackoverflow.com/questions/16144115/makefile-remove-duplicate-words-without-sorting
-define uniq =
+define uniq
   $(eval seen :=)
   $(foreach _,$1,$(if $(filter $_,$(seen)),,$(eval seen += $_)))
   $(seen)