[gdb/testsuite] Fix timeout in gdb.tui/empty.exp

[gdb/testsuite] Fix timeout in gdb.tui/empty.exp

In test-case gdb.tui/empty.exp we run into:
...
WARNING: timeout in accept_gdb_output
PASS: gdb.tui/empty.exp: src: 90x40: box 1
...

We timeout here in Term::resize:
...
	# Due to the strange column resizing behavior, and because we
	# don't care about this intermediate resize, we don't check
	# the size here.
	wait_for "@@ resize done $_resize_count"
...
because the string we're trying to match is split over two lines:
...
25 -----------------------------------------------------------------------------+No
26 ne No process In:                                               L??   PC: ?? @@
27 resize done 0, size = 79x40
28 (gdb)
...

Fix this by dropping the "@@ " prefix.

Tested on x86_64-linux.