fix -Wdeprecated-copy on clang (BaseError copy assignment)

2bbe3efd1¹ added the -Wdeprecated-copy warning, and fixed the instances
of it which GCC warned about, in HintFmt and ref<T>. However, when
building with Clang, there is an additional deprecated-copy warning in
BaseError. This commit explicitly defaults the copy assignment operator
for BaseError and silences this warning.

1: 2bbe3efd16
Change-Id: I50aa4a7ab1a7aae5d7b31f765994abd3db06379d

src/libutil/error.hh

@@ -110,6 +110,8 @@ protected:
 public:
     BaseError(const BaseError &) = default;
 
+    BaseError & operator=(BaseError const & rhs) = default;
+
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
         : err { .level = lvlError, .msg = HintFmt(args...), .status = status }