gdb: Silence -Wformat-nonliteral warning with clang

gdb: Silence -Wformat-nonliteral warning with clang

We get this warning when building with clang:

      CXX    ui-out.o
    /home/smarchi/src/binutils-gdb/gdb/ui-out.c:590:22: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
      do_message (style, format, args);
                         ^~~~~~

This can be considered a legitimate warning, as call_do_message's format
parameter is not marked as a format string.  Therefore, we should
normally mark the call_do_message method with the `format` attribute.
However, doing so just moves (and multiplies) the problem, as all the
uses of call_do_message in the vmessage method now warn.  If we wanted
to continue on that path, we should silence the warning for each of
them, as a way of telling the compiler "it's ok, we know what we are
doing".

But since call_do_message is really just vmessage's little helper, it's
simpler to just silence the warning at that single point.

gdb/ChangeLog:

	* ui-out.c (ui_out::call_do_message): Silence
	-Wformat-nonliteral warning.

Change-Id: I58ad41793448f38835c5d6ba7b9e5c4dd8df260f