Add -Wextra-semi -Wundefined-func-template to the default CXXFLAGS

And fix the resulting compilation errors.

Change-Id: I16f679836a0cf2a3ff5dfcf6f6b67bfbfe4cdbd7
Refs: #4248
diff --git a/src/signature-info.hpp b/src/signature-info.hpp
index 7ac8404..af6908d 100644
--- a/src/signature-info.hpp
+++ b/src/signature-info.hpp
@@ -162,6 +162,8 @@
   operator<<(std::ostream& os, const SignatureInfo& info);
 };
 
+NDN_CXX_DECLARE_WIRE_ENCODE_INSTANTIATIONS(SignatureInfo);
+
 bool
 operator==(const SignatureInfo& lhs, const SignatureInfo& rhs);