gdbsupport: add format attribute to print_xml_feature::add_line
Fixes this clang error:
CXX tdesc.o
/home/smarchi/src/binutils-gdb/gdbsupport/tdesc.cc:444:25: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
string_vappendf (tmp, fmt, ap);
^~~
There is already a but about GCC not emitting this warning: