Makefile: run vendor in contrib/tetragon-rthooks on protogen

contrib/tetragon-rthooks module is dependent on the API, so we can
immediately run vendor on proto generation so that users don't need to
realize it later and run make vendor independently.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>

Makefile

@@ -380,6 +380,7 @@ protogen: protoc-gen-go-tetragon ## Generate code based on .proto files.
 	$(GO) mod tidy
 	$(GO) mod vendor
 	$(GO) mod verify
+	$(MAKE) -C contrib/tetragon-rthooks vendor
 
 .PHONY: protoc-gen-go-tetragon
 protoc-gen-go-tetragon: