Fix trailing slashes in the Servo makefile.

This fixes a build issue where GNU Make 4.0 would stop with:

    No rule to make target '…/rust-http/src/http/generated/read_method.rs',
    needed by 'libhttp.dummy'.  Stop.

Apparently other people did not have this issue,
it probably only affects some Make versions.

Author: SimonSapin

Makefile.servo.in

@@ -18,7 +18,7 @@ codegen: $(wildcard $(VPATH)/src/codegen/*.rs)
 $(VPATH)/src/http/generated:
 	mkdir -p $(VPATH)/src/http/generated
 
-$(VPATH)/src/http/generated/%.rs: codegen $(VPATH)/src/http/generated/
+$(VPATH)/src/http/generated/%.rs: codegen $(VPATH)/src/http/generated
 	./codegen $(patsubst $(VPATH)/src/http/generated/%,%,$@) $(VPATH)/src/http/generated/
 
 libhttp.dummy: $(libhttp_files)
@@ -42,7 +42,7 @@ clean-tests:
 	rm -f tests
 
 clean: clean-tests
-	rm -rf $(VPATH)src/http/generated/ codegen
+	rm -rf $(VPATH)/src/http/generated/ codegen
 	rm -rf libhttp.dummy
 	rm -f *.so *.dylib *.dll