Fix make warning for autogenerated-casts.c override

Use $(filter-out) to eliminate autogenerated-casts.c from the general
recipe, so that make does not warn when it encounters a specific recipe
for autogenerated-casts.c.

Author: Kevin Pyle

Makefile

@@ -195,7 +195,7 @@ autogenerated-casts.h: autogenerated-casts.c
 
 # Generate to a temporary name, so that a failure in the script does not
 # leave behind an up-to-date incomplete file.
-$(PLUGIN_GENERATED_SOURCE_FILES): autogenerated-%.c: $(addprefix $(srcdir),generate-%-c.py $(GENERATOR_DEPS))
+$(filter-out autogenerated-casts.c,$(PLUGIN_GENERATED_SOURCE_FILES)): autogenerated-%.c: $(addprefix $(srcdir),generate-%-c.py $(GENERATOR_DEPS))
 	$(PYTHON) $< > "$@.part" || { rc=$$?; $(RM) "$@.part"; exit $$rc; }
 	mv -f "$@.part" "$@"