[banjo] Fix function prototypes in ddktl static_asserts

TEST: manual examination of compiler errors generated by these asserts
Change-Id: Id386222ea0899b0f8b60e0a29de4e95228585e03
diff --git a/system/host/banjo/lib/ddk_generator.cpp b/system/host/banjo/lib/ddk_generator.cpp
index 0fb2903..9694684 100644
--- a/system/host/banjo/lib/ddk_generator.cpp
+++ b/system/host/banjo/lib/ddk_generator.cpp
@@ -1744,7 +1744,7 @@
         file_ << kIndent << kIndent << "\"" << cc_name << " subclasses must implement \"\n";
         file_ << kIndent << kIndent << "\"";
         EmitProtocolMethodDecl(&file_, method_info.protocol_name, input, output);
-        file_ << "\");\n";
+        file_ << ");\");\n";
     }
     file_ << "}\n";
     EmitBlank(&file_);