Only print "no debugging symbols" message once

Only print "no debugging symbols" message once

The "no debugging symbols" message can be confusing in some cases, for
example when gdb finds separate debug info for an objfile, but the
separate debug info does not contain symbols.

For example:

    (gdb) file /bin/ls
    Reading symbols from /bin/ls...
    Reading symbols from .gnu_debugdata for /usr/bin/ls...
    (No debugging symbols found in .gnu_debugdata for /usr/bin/ls)
    (No debugging symbols found in /bin/ls)

Here, I think the second "no debugging symbols" message is redundant
and confusing.

This patch changes gdb to only emit this message when the objfile in
question does not have a separate debug file.  So, in the example
above, the output would now read:

    (gdb) file /bin/ls
    Reading symbols from /bin/ls...
    Reading symbols from .gnu_debugdata for /usr/bin/ls...
    (No debugging symbols found in .gnu_debugdata for /usr/bin/ls)

2018-10-04  Tom Tromey  <tom@tromey.com>

	* symfile.c (symbol_file_add_with_addrs): Do not print "no
	debugging symbols" message if there is a separate debug objfile.