build: clean-go: Do not warn about *.go files in "test-tmp".

* Makefile.am (clean-go): Ignore "test-tmp" directory.

Makefile.am

@@ -318,7 +318,7 @@ nobase_nodist_guileobject_DATA = $(GOBJECTS)
 # Handy way to remove the .go files without removing all the rest.
 clean-go:
 	-$(RM) -f $(GOBJECTS)
-	@find . -name '*.go' -print | \
+	@find . -path ./test-tmp -prune -o -name '*.go' -print | \
 	  if test -t 1; then \
 	    xargs -r echo -e "\033[31mwarning:\033[0m stray .go files:"; \
 	  else \