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:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82206

gdbsupport/ChangeLog:

	* tdesc.h (class print_xml_feature) <add_line>: Add
	ATTRIBUTE_PRINTF.

Change-Id: I7014075e83717f6d7e19d044a3675ff9981ebe17