gdbsupport/tdesc: print enum size attribute

According to gdb online docs[1], XML target description enum types
have both name and size attributes.  Currently GDB does not print the
size attribute.  This commit fixes this.  This change will be visible
in the output of the command `maint print xml-tdesc`.

There are other bugs with the print of enum types in XML target
descriptions, the next commit will fix these and include a test that
covers this patch.

[1] https://sourceware.org/gdb/current/onlinedocs/gdb/Enum-Target-Types.html#Enum-Target-Types

gdbsupport/ChangeLog:

	* tdesc.cc (print_xml_feature::visit): Print enum size attribute.