Add missing assignment operators

Add missing assignment operators

This was detected by compiling with GCC 9.0.1

	* include/abg-interned-str.h (interned_string::operator=): Define
	assignment operator.
	* include/abg-ir.h
	({location, enum_type_decl::enumerator}::operator=): Declare
	assignment operator.
	* src/abg-ir.cc (enum_type_decl::enumerator::operator=): Define
	assignment operator.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>